Materials and methods for producing metal nanocomposites, and metal nanocomposites obtained therefrom

ABSTRACT

Some variations provide a metal matrix nanocomposite composition comprising metal-containing microparticles and nanoparticles, wherein the nanoparticles are chemically and/or physically disposed on surfaces of the microparticles, and wherein the nanoparticles are consolidated in a three-dimensional architecture throughout the composition. The composition may serve as an ingot for producing a metal matrix nanocomposite. Other variations provide a functionally graded metal matrix nanocomposite comprising a metal-matrix phase and a reinforcement phase containing nanoparticles, wherein the nanocomposite contains a gradient in concentration of the nanoparticles. This nanocomposite may be or be converted into a master alloy. Other variations provide methods of making a metal matrix nanocomposite, methods of making a functionally graded metal matrix nanocomposite, and methods of making a master alloy metal matrix nanocomposite. The metal matrix nanocomposite may have a cast microstructure. The methods disclosed enable various loadings of nanoparticles in metal matrix nanocomposites with a wide variety of compositions.

PRIORITY DATA

This patent application is a divisional application of U.S. patentapplication Ser. No. 17/076,803, filed on Oct. 22, 2020, which is adivisional application of U.S. Pat. No. 10,865,464, issued on Dec. 15,2020, which claims priority to U.S. Provisional Patent App. No.62/422,925, filed on Nov. 16, 2016; U.S. Provisional Patent App. No.62/422,930, filed on Nov. 16, 2016; and U.S. Provisional Patent App. No.62/422,940, filed on Nov. 16, 2016, each of which is hereby incorporatedby reference herein.

FIELD OF THE INVENTION

The present invention generally relates to metal matrix nanocomposites,and methods of making and using the same.

BACKGROUND OF THE INVENTION

Metal matrix nanocomposite materials have attracted considerableattention due to their ability to offer unusual combinations ofstiffness, strength to weight ratio, high-temperature performance, andhardness. There is a wide variety of commercial uses of metal matrixnanocomposites, including high-wear-resistant alloy systems,creep-resistant alloys, high-temperature alloys with improved mechanicalproperties, and radiation-tolerant alloys.

Currently, there are difficulties in making metal matrix nanocompositesincluding processing costs and high capital investment for equipment toprocess materials. There are very few effective methods of maintaining ahomogenously dispersed nanoparticle reinforcement phase in a metalmatrix, especially in melt processing. Reinforcement phase reactivityand particulate agglomeration of nanoscale reinforcement limit thestrengthening effects in currently produced metal matrix nanocomposites.

There is a desire for lower-cost routes to produce thesehigh-performance nanocomposites, including low-volume-fractionnanocomposites as well as high-volume-fraction nanocomposites (i.e.nanocomposites containing various concentrations of nanoparticles).

Current methods for producing low-volume-fraction nanocomposites arelimited to in-situ reaction mechanisms in highly specific alloy systems.These include oxide dispersion-strengthened copper and steels in whichoxide formers such as aluminum are incorporated into the alloy in orderto scavenge dissolved oxygen and form nano-oxides. Similar techniquescan be used for nitrides and carbides. These techniques requiresubstantial atmosphere control and temperature control to ensure thatthe nucleation rate within the material is stable, so that significantcoarsening does not occur. The materials are therefore extremelyexpensive and geometry-limited. Due to the kinetics of diffusion,nucleation, and growth, geometries must be relatively uniform and thinto allow uniform composite formation. Thick sections take much longerfor the center of the material to begin nucleating oxides, nitrides, orcarbides. Thus the material cannot be made with uniform propertiesthrough the thickness.

High volume loading of nanoscale reinforcements ex situ is limited tofew processes and none with the capability of producing geometricallycomplex shapes and at a low cost. Current melt processing methods suchas shear mixing or ultrasonic processing of metal matrix nanocompositessuffer from a limited availability of compatible materials due toreactivity and dispersion issues. These methods are capable ofdispersing low volume percentages of certain reinforcement phases;however, complications arise at higher reinforcement volume loadingpercentages as the effects of dispersion become more localized and lesseffective at higher melt viscosities.

Current methods to produce high-volume-fraction nanocomposites rely on avariety of high-cost methods to incorporate the nanoparticles. These canbe incorporated using high-energy ball milling which physically forcesthe nanomaterials into the matrix material, and then the remainingmaterial is processed into a part. This requires batch processing. Also,very large high-energy ball mills present both cost and safety barriers.Nanomaterials may also be incorporated in the melt, but distribution ofthe nanomaterials can be difficult due to the surface energiesassociated with liquid metal. Ultrasonic mixing or high-shear mixing canbe effective, but they are size-limited and require manipulation ofmolten metal, which again presents cost and safety barriers. Anothermethod utilizes the semisolid state in which particles are incorporatedthrough a friction stir process. This is highly localized and notimmediately scalable.

There is also a desire for functionally graded metal matrixnanocomposites that contain some type of functional gradient (e.g.,nanoparticle concentration) within the nanocomposite. Functionallygraded metal matrix nanocomposites have not yet been successfullyproduced with a conventional melt processing method, due in large partto the high reactivity of reinforcement phase in a metal melt.

Homogeneously dispersed metal matrix nanocomposites have been producedusing high-energy ultrasonication to enhance dispersion and wettingcharacteristics of nanoparticles in metal melts. This technique relieson cavitation of gases and acoustically driven mixing of particulateadded ex situ into the melt. Functionally graded materials have not beenproduced in this manner due to particulate instability in the lengthyprocessing needed for full dispersion. The ultrasonication process isinherently limited to particulates that are highly stable in the moltenmatrix during processing and solidification.

Additionally, wettability of many potential reinforcement phasesdisqualifies them from being used in ex-situ melt processing techniqueswhere inclusion of the particulate phase into the melt is highlydependent on wettability of the particulate phase with the metal matrix.Particulate-matrix compatibility requirements inhibit the availabilityof acceptable reinforcement phases in metal matrix nanocompositeproduction. Additionally, the loading of high volumes of nanoparticlesbecomes problematic in ultrasonic dispersion techniques as the effect ofdispersion becomes more localized at high melt viscosities induced byhigh-volume loading of a reinforcement phase.

Friction stir processing can produce metal matrix nanocomposites bydriving the particulate phase into the metal through the semisolidcreated by friction with a probe. Friction stir processing has been usedto produce functionally graded metal matrix nanocomposites; however,this process is geometrically constrained and cannot be used with metalsand alloys without a viable semisolid processing region. Friction stirprocessing can alter the microstructural integrity of the bulk material,as large amounts of heat from the friction produced affect thesurrounding microstructures near the processing zone. Also, thickness ofparts produced in friction stir processing is limited to a few inches.Scaling of friction stir processing is very limited and production ofhigh volumes of metal matrix nanocomposites is not feasible.

The current high cost, lack of availability, and lack of alloy diversitycurrently available for nanocomposites is a testament to the difficultyin producing these materials.

Conventional melt processing techniques such as liquid stir processing,semisolid stir processing, and ultrasonic processing are capable ofdispersing low volumes of reinforcement phase which are nonreactive withthe metal melt. What is desired is a method that enables both highvolume loading and reactive reinforcement phases.

What is also sought is a method of producing a functionally graded metalmatrix nanocomposite that is amenable to conventional melt processingtechniques, with a wide variety of acceptable materials that may beused. A method is needed to produce a functionally graded metal matrixnanocomposite in which processing times are limited so thatnanoparticles do not degrade during processing.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, aswill now be summarized and then further described in detail below.

Some variations of the invention provide a composition comprisingmetal-containing microparticles and nanoparticles, wherein thenanoparticles are chemically and/or physically disposed on surfaces ofthe microparticles, and wherein the nanoparticles are consolidated in athree-dimensional architecture throughout the composition.

In some embodiments, the composition is an ingot for producing a metalnanocomposite. In other embodiments, the composition itself is a metalnanocomposite.

The microparticles may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof,for example. The nanoparticles may contain a compound selected from thegroup consisting of metals, ceramics, cermets, intermetallic alloys,oxides, carbides, nitrides, borides, polymers, carbon, and combinationsthereof, for example. In certain embodiments, the microparticles containAl, Si, and Mg (e.g., alloy AlSi10Mg), and the nanoparticles containtungsten carbide (WC).

In some embodiments, the microparticles have an average microparticlesize from about 1 micron to about 1 centimeter. In some embodiments, thenanoparticles have an average nanoparticle size from about 1 nanometerto about 1000 nanometers.

The composition may contain from about 10 wt % to about 99.9 wt % ofmicroparticles. In these or other embodiments, the composition containsfrom about 0.1 wt % to about 10 wt % of the nanoparticles.

Other variations of the invention provide a functionally graded metalmatrix nanocomposite comprising a metal-matrix phase and a firstreinforcement phase containing first nanoparticles, wherein thenanocomposite contains a gradient in concentration of the firstnanoparticles through at least one dimension of the nanocomposite. Thegradient in concentration of the nanoparticles may be present in thenanocomposite over a length scale of at least 100 microns. Thenanocomposite has a cast microstructure, in some embodiments.

In some embodiments, the nanocomposite is a master alloy. Themetal-matrix phase may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.The first nanoparticles may contain a compound selected from the groupconsisting of metals, ceramics, cermets, intermetallic alloys, oxides,carbides, nitrides, borides, polymers, carbon, and combinations thereof.In some embodiments, the metal-matrix phase contains Al, Si, and Mg, andthe first nanoparticles contain tungsten carbide (WC).

The first nanoparticles may have an average particle size from about 1nanometer to about 1000 nanometers. Some or all of the firstnanoparticles may be agglomerated such that the effective particle sizein the nanoparticle phase is larger than 1000 nanometers, in someembodiments.

The nanocomposite may contain from about 10 wt % to about 99.9 wt % ofthe metal-matrix phase, for example. The nanocomposite may contain fromabout 0.1 wt % to about 10 wt % of the first nanoparticles, for example.

In some embodiments, the nanocomposite further comprises secondnanoparticles in the first reinforcement phase and/or in a secondreinforcement phase.

In some embodiments, the metal-matrix phase and the first reinforcementphase are each dispersed throughout the nanocomposite. In these or otherembodiments, the metal-matrix phase and the first reinforcement phaseare disposed in a layered configuration within the nanocomposite,wherein the layered configuration includes at least a first layercomprising the first nanoparticles and at least a second layercomprising the metal-matrix phase.

The nanocomposite may be present in an object that has at least onedimension of 100 microns or greater, such as 1 millimeter or greater.

Certain variations of the invention provide a functionally graded metalmatrix nanocomposite comprising a metal-matrix phase containing Al, Si,and Mg and a reinforcement phase containing W and C, wherein thenanocomposite contains a gradient in concentration of the reinforcementphase through at least one dimension of the nanocomposite. Thenanocomposite may have a cast microstructure.

The metal-matrix phase contains aluminum alloy AlSi10Mg, in certainembodiments. The reinforcement phase contains tungsten carbide (WC), incertain embodiments. In some embodiments, the metal-matrix phase and thereinforcement phase are disposed in a layered configuration within thenanocomposite, wherein the layered configuration includes a first layercomprising the W and C and the Al, Si, and Mg, and a second layercomprising the Al, Si, and Mg.

Other variations of the invention provide a method of making a metalnanocomposite, the method comprising:

(a) providing a precursor composition comprising metal-containingmicroparticles and nanoparticles, wherein the nanoparticles arechemically and/or physically disposed on surfaces of the microparticles;

(b) consolidating the precursor composition into an intermediatecomposition comprising the metal-containing microparticles and thenanoparticles, wherein the nanoparticles are consolidated in athree-dimensional architecture throughout the intermediate composition;and

(c) processing the intermediate composition to convert the intermediatecomposition into a metal nanocomposite.

In some embodiments, the precursor composition is in powder form. Insome embodiments, the intermediate composition is in ingot form. Thefinal nanocomposite may have a cast microstructure, in some embodiments.

The microparticles may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.The nanoparticles may contain a compound selected from the groupconsisting of metals, ceramics, cermets, intermetallic alloys, oxides,carbides, nitrides, borides, polymers, carbon, and combinations thereof.

In various embodiments, step (b) includes pressing, binding, sintering,or a combination thereof.

In various embodiments, step (c) includes pressing, sintering, mixing,dispersing, friction stir welding, extrusion, binding, melting,semi-solid melting, capacitive discharge sintering, casting, or acombination thereof.

In some embodiments, the metal phase and the first reinforcement phaseare each dispersed throughout the nanocomposite. In these or otherembodiments, the metal phase and the first reinforcement phase aredisposed in a layered configuration within the nanocomposite, whereinthe layered configuration includes at least a first layer comprising thenanoparticles and at least a second layer comprising the metal phase.

Other variations provide a method of making a functionally graded metalmatrix nanocomposite, the method comprising:

(a) providing a precursor composition (e.g., powder) comprisingmetal-containing microparticles and nanoparticles, wherein thenanoparticles are chemically and/or physically disposed on surfaces ofthe microparticles;

(b) consolidating the precursor composition into an intermediatecomposition (e.g., ingot) comprising the metal-containing microparticlesand the nanoparticles, wherein the nanoparticles are consolidated in athree-dimensional architecture throughout the intermediate composition;

(c) melting the intermediate composition to form a melt, wherein themelt segregates into a first phase comprising the metal-containingmicroparticles and a second phase comprising the nanoparticles; and

(d) solidifying the melt to obtain a metal matrix nanocomposite with agradient in concentration of the nanoparticles through at least onedimension of the nanocomposite.

The microparticles may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.The nanoparticles may contain a compound selected from the groupconsisting of metals, ceramics, cermets, intermetallic alloys, oxides,carbides, nitrides, borides, polymers, carbon, and combinations thereof.In some embodiments, the microparticles contain Al, Si, and Mg, and thenanoparticles contain tungsten carbide (WC).

In various embodiments, step (b) includes pressing, binding, sintering,or a combination thereof.

In various embodiments, step (c) includes pressing, sintering, mixing,dispersing, friction stir welding, extrusion, binding, melting,semi-solid melting, capacitive discharge sintering, casting, or acombination thereof. Step (c) may also include holding the melt for aneffective dwell time to cause density-driven segregation of the firstphase from the second phase. The dwell time may be selected from about 1minute to about 8 hours, for example. In some embodiments, step (c)includes exposing the melt to an external force selected fromgravitational, centrifugal, mechanical, electromagnetic, or acombination thereof.

Step (d) may include directional solidification of the melt. In someembodiments, the nanocomposite has a cast microstructure. Themetal-matrix phase and the first reinforcement phase may be eachdispersed throughout the nanocomposite. In these or other embodiments,the metal-matrix phase and the first reinforcement phase are disposed ina layered configuration within the nanocomposite, wherein the layeredconfiguration includes at least a first layer comprising thenanoparticles and at least a second layer comprising the metal-matrixphase.

The gradient in concentration of the nanoparticles may be present in thenanocomposite over a length scale of at least 100 microns.

Other variations of the invention provide a method of making a masteralloy metal matrix nanocomposite, the method comprising:

(a) providing an ingot composition comprising metal-containingmicroparticles and nanoparticles, wherein the nanoparticles arechemically and/or physically disposed on surfaces of the microparticles,and wherein the nanoparticles are consolidated in a three-dimensionalarchitecture throughout the ingot composition;

(b) melting the ingot composition to form a melt, wherein the meltsegregates into a first phase comprising the metal-containingmicroparticles and a second phase comprising the nanoparticles;

(c) solidifying the melt to obtain a metal matrix nanocomposite with agradient in concentration of the nanoparticles through at least onedimension of the nanocomposite; and

(d) removing a fraction of the metal matrix nanocomposite containing alower concentration of the nanoparticles compared to the remainder ofthe metal matrix nanocomposite, thereby producing a master alloy metalmatrix nanocomposite.

The microparticles may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.The nanoparticles may contain a compound selected from the groupconsisting of metals, ceramics, cermets, intermetallic alloys, oxides,carbides, nitrides, borides, polymers, carbon, and combinations thereof.In certain embodiments, the microparticles contain Al, Si, and Mg, andthe nanoparticles contain tungsten carbide (WC).

Step (b) may further include pressing, sintering, mixing, dispersing,friction stir welding, extrusion, binding, capacitive dischargesintering, casting, or a combination thereof. Step (b) may includeholding the melt for an effective dwell time (e.g., about 1 minute to 8hours) to cause density-driven segregation of the first phase from thesecond phase. Optionally, step (b) may include exposing the melt to anexternal force selected from gravitational, centrifugal, mechanical,electromagnetic, or a combination thereof.

Step (c) may include directional solidification of the melt. In someembodiments, the metal matrix nanocomposite in step (c) is characterizedby a cast microstructure. The gradient in concentration of the firstnanoparticles may be present in the metal matrix nanocomposite over alength scale of at least 100 microns.

In some embodiments, the metal-matrix phase and the first reinforcementphase are each dispersed throughout the metal matrix nanocomposite. Inthese or other embodiments, the metal-matrix phase and the firstreinforcement phase are disposed in a layered configuration within themetal matrix nanocomposite, wherein the layered configuration includesat least a first layer comprising the nanoparticles and at least asecond layer comprising the metal-matrix phase.

Step (d) may include includes machining, ablation, reaction,dissolution, evaporation, selective melting, or a combination thereof.In certain embodiments, step (d) provides two distinct master alloymetal matrix nanocomposites.

The final master alloy metal matrix nanocomposite(s) may have a castmicrostructure, in some embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The schematic drawings herein represent functionalization patterns andmicrostructures which may be achieved in embodiments of the invention.These drawings should not be construed as limiting in any way. It isalso noted that illustrations contained in the drawings are not drawn toscale and various degrees of zooming-in are employed for purposes ofunderstanding these embodiments.

FIG. 1 depicts some embodiments in which a functionalized powdercontaining metal microparticles coated with nanoparticles is convertedto an ingot (or other material) with the nanoparticles oriented in athree-dimensional structure.

FIG. 2 depicts some embodiments in which a functionalized powdercontaining metal microparticles coated with nanoparticles is convertedto a melt or ingot (or other material), and then the nanoparticles reactin the melt to form a new distributed phase containing nanoparticles.

FIG. 3 depicts some embodiments starting with a functionalized powdercontaining metal microparticles coated with two types of nanoparticles,which are differently chemically and/or physically, and then thefunctionalized powder is converted to a melt or ingot (or othermaterial) containing nanoparticles distributed in the metal phase.

FIG. 4 depicts some embodiments starting with a functionalized powdercontaining metal microparticles coated with two types of nanoparticles,which are differently chemically and/or physically, and then one of thenanoparticles reacts while the other does not within the metal phase.

FIG. 5 depicts some embodiments starting with nanoparticlespredistributed in a metal matrix, such as in an ingot, withdensity-driven phase segregation in which nanoparticles migrate towardthe surface, followed by solidification, resulting in a functionallygraded metal matrix nanocomposite.

FIG. 6 depicts some embodiments starting with nanoparticlespredistributed in a metal matrix, such as in an ingot, withdensity-driven phase segregation in which nanoparticles migrate awayfrom the surface, followed by solidification, resulting in afunctionally graded metal matrix nanocomposite.

FIG. 7 depicts some embodiments starting with codispersed nanoparticlespredistributed in a metal matrix, such as in an ingot, withdensity-driven phase segregation in which some nanoparticles migrateaway from the surface while other nanoparticles migrate toward thesurface, followed by solidification, resulting in a functionally gradedmetal matrix nanocomposite.

FIG. 8 depicts some embodiments starting with codispersed nanoparticlespredistributed in a metal matrix, such as in an ingot, withdensity-driven phase segregation in which nanoparticles migrate awayfrom the surface, followed by solidification, resulting in afunctionally graded metal matrix nanocomposite.

FIG. 9 depicts some embodiments starting with codispersed nanoparticlespredistributed in a metal matrix, such as in an ingot, withdensity-driven phase segregation in which nanoparticles migrate towardthe surface, followed by solidification, resulting in a functionallygraded metal matrix nanocomposite.

FIG. 10 is an SEM image of a cross-section (side view) of an exemplaryAlSi10Mg—WC functionally graded metal matrix nanocomposite, according toExample 1 herein.

FIG. 11 is an SEM image of a cross-section (side view) of an exemplaryAlSi10Mg—WC master alloy metal matrix nanocomposite, according toExample 2 herein.

FIG. 12 depicts some embodiments to produce a master alloy metal matrixnanocomposite enriched with nanoparticles in a metal matrix, by firstproducing a functionally graded metal matrix nanocomposite and thenremoving a phase of material containing a relatively low volume fractionof nanoparticles.

FIG. 13 depicts some embodiments to produce a master alloy metal matrixnanocomposite enriched with nanoparticles in a metal matrix, by firstproducing a functionally graded metal matrix nanocomposite and thenremoving a phase of material containing a relatively low volume fractionof nanoparticles.

FIG. 14 depicts some embodiments to produce a master alloy metal matrixnanocomposite enriched with two types of nanoparticles in a metalmatrix, by first producing a functionally graded metal matrixnanocomposite and then removing a phase of material containing arelatively low volume fraction of both types of nanoparticles.

FIG. 15 depicts some embodiments to produce two distinct master alloymetal matrix nanocomposites enriched with different types ofnanoparticles in a metal matrix, by first producing a functionallygraded metal matrix nanocomposite and then removing a phase of materialcontaining a relatively low volume fraction of both types ofnanoparticles.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The compositions, structures, systems, and methods of the presentinvention will be described in detail by reference to variousnon-limiting embodiments.

This description will enable one skilled in the art to make and use theinvention, and it describes several embodiments, adaptations,variations, alternatives, and uses of the invention. These and otherembodiments, features, and advantages of the present invention willbecome more apparent to those skilled in the art when taken withreference to the following detailed description of the invention inconjunction with the accompanying drawings.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contextclearly indicates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as is commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

Unless otherwise indicated, all numbers expressing conditions,concentrations, dimensions, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending at least upona specific analytical technique.

The term “comprising,” which is synonymous with “including,”“containing,” or “characterized by” is inclusive or open-ended and doesnot exclude additional, unrecited elements or method steps. “Comprising”is a term of art used in claim language which means that the named claimelements are essential, but other claim elements may be added and stillform a construct within the scope of the claim.

As used herein, the phrase “consisting of” excludes any element, step,or ingredient not specified in the claim. When the phrase “consists of”(or variations thereof) appears in a clause of the body of a claim,rather than immediately following the preamble, it limits only theelement set forth in that clause; other elements are not excluded fromthe claim as a whole. As used herein, the phrase “consisting essentiallyof” limits the scope of a claim to the specified elements or methodsteps, plus those that do not materially affect the basis and novelcharacteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consistingessentially of,” where one of these three terms is used herein, thepresently disclosed and claimed subject matter may include the use ofeither of the other two terms. Thus in some embodiments not otherwiseexplicitly recited, any instance of “comprising” may be replaced by“consisting of” or, alternatively, by “consisting essentially of.”

Variations of this invention are predicated on the control ofsolidification of powder materials. Controlling solidification can havea drastic impact on microstructure and thus material properties (e.g.strength and toughness). In some cases faster solidification isdesirable; while in other cases slow solidification may produce thedesired microstructure. In certain cases it is not desirable to fullymelt the powder; but rather to melt and solidify only at the powdersurface. This invention provides routes to control—in both time andspace—solidification in materials, utilizing surface functionalizationof the primary powder being processed.

Some variations provide routes to controlled solidification of materialswhich are generally difficult or impossible to process otherwise. Theprinciples disclosed herein may be applied to additive manufacturing aswell as joining techniques, such as welding. Certain unweldable metals,such as high-strength aluminum alloys (e.g., aluminum alloys 7075, 7050,or 2199) would be excellent candidates for additive manufacturing butnormally suffer from hot cracking. The methods disclosed herein allowthese alloys to be processed with significantly reduced crackingtendency.

Proper control of solidification can lead to greater part reliabilityand enhanced yield. Some embodiments of the invention provide powdermetallurgy—processed parts that are equivalent to machined parts. Someembodiments provide corrosion-resistant surface coatings that are formedduring the part fabrication instead of as an extra step.

This disclosure describes control of nucleation and growth kineticswithin the structure independent of, or in conjunction with, thermalinput. This disclosure describes methods which incorporate phase andstructure control to generate three-dimensional microstructuralarchitecture. Methods for inclusion/contaminant removal are provided, aswell as development of composite structures.

Variations of this invention are premised on controlling solidificationthrough limiting or increasing thermal conductivity and/or radiationwith the surroundings, utilizing enthalpies of formation and varyingheat capacities to control thermal loads during solidification, and/orutilizing surface tension to control entrapment of desired species—orrejection of undesired species—in the final solidification product.

Some variations provide methods to control nanoparticle (ormicroparticle)/material segregation. When rapid solidificationtechniques are applied to powder processing, a unique microstructure maybe developed. Likewise, the configuration of the nanoparticles ormicroparticles around the particles prior to melting may introduce athree-dimensional nanoparticle architecture within the overallmicrostructure.

Embodiments of this invention provide three-dimensional nanoparticlearchitectures within metal microstructures. Not wishing to be bound bytheory, these architectures may significantly improve the materialproperties by impeding, blocking, or redirecting dislocation motion inspecific directions. This discovery may be used to control failuremechanisms beyond prior-art isotropic or anisotropic materials.

The present invention is not limited to metallic materials and canprovide similar benefits with a significantly less difficult, morerepeatable, and energy-efficient production method. The semi-passivenature of the process typically requires no alteration of existingtooling and can be employed in existing manufacturing settings.

Production of Metal Matrix Nanocomposites

Some variations of the present invention provide starting materials ormaterial systems useful for producing metal matrix nanocomposites, andmetal matrix nanocomposites obtained therefrom. A “metal matrixnanocomposite” (or “MMNC”) or equivalently “metal nanocomposite” is ametal-containing material with greater than 0.1 wt % nanoparticlesdistributed in a metal matrix or otherwise within the metal-containingmaterial.

Nanocomposites have been shown to exhibit enhanced mechanical strengthdue to the ability to impede dislocation motion. This ability is notlimited to room temperature and can improve a material'shigh-temperature strength and creep resistance. Nanocomposites can alsoimprove wear and fouling resistance in certain sliding and high-frictionenvironments. However, nanocomposites have been heretofore difficult toproduce and therefore their use has been limited.

Variations of this invention are premised on the discovery of a pathwayto produce metal matrix nanocomposites of arbitrary composition and withcontrol of nanoparticle volume fraction. Starting with functionalizedmetal feedstocks as described later in the specification (sectionentitled “Functionalized Metal Feedstocks for Producing Metal MatrixNanocomposites”), a low or high volume fraction of nanoparticles may beachieved. There may be a uniform or non-uniform distribution ofnanoparticles within the matrix, by utilizing conventional, low-costpowder metallurgy approaches and ingot processing.

A “functionalized metal” or “functionalized metal feedstock” comprises ametal microparticle with one or more different nanoparticles assembledon the surface. The nanoparticles are typically a different compositionthan the base micropowder.

The nanoparticles are chemically and/or physically disposed on surfacesof the microparticles. That is, the nanoparticles may be attached usingelectrostatic forces, Van der Waals forces, chemical bonds, mechanicalbonds, and/or any other force(s). A chemical bond is the force thatholds atoms together in a molecule or compound. Electrostatic and Vander Waals forces are examples of physical forces that can cause bonding.A mechanical bond is a bond that arises when molecular entities becomeentangled in space. Typically, chemical bonds are stronger than physicalbonds.

Nanoparticles of interest include carbides, nitrides, borides, oxides,intermetallics, or other materials which upon processing may form one ormore of the aforementioned materials. The size, shape, and compositionof the nanoparticles may vary widely. The nanoparticles typically havean average nanoparticle size from about 1 nanometer to about 1000nanometers, such as about 250 nanometers or less. In some embodiments,strength increases are favored by smaller nanoparticles. In someapplications, the material may be processed with larger constituentparticles (such as about 250-1000 nanometers or larger) to produce adesirable material.

Some variations provide a cost-effective route to producing large-scaleraw materials for the production of metal nanocomposites. Certainembodiments utilize functionalized powder feedstocks as described inU.S. patent application Ser. No. 15/209,903, filed on Jul. 14, 2016,which is hereby incorporated by reference herein. The present disclosureis not limited to those functionalized powders.

Some variations of the invention provide a metal matrix nanocompositecomposition comprising metal-containing microparticles andnanoparticles, wherein the nanoparticles are chemically and/orphysically disposed on surfaces of the microparticles, and wherein thenanoparticles are consolidated in a three-dimensional architecturethroughout the composition.

A “three-dimensional architecture” means that the nanoparticles are notrandomly distributed throughout the metal matrix nanocomposite. Rather,in a three-dimensional architecture of nanoparticles, there is someregularity in spacing between nanoparticles, in space (threedimensions). The average spacing between nanoparticles may vary, such asfrom about 1 nanoparticle diameter to about 100 nanoparticle diametersor more, depending on the nanoparticle concentration in the material.

In some embodiments, the three-dimensional architecture of nanoparticlesin the metal matrix nanocomposite is correlated to the distribution ofnanoparticles within the starting composition (functionalmicroparticles, i.e. metal-containing microparticles with nanoparticleson surfaces). An illustration of this is shown in FIG. 1. Such athree-dimensional architecture of nanoparticles is possible when thekinetics during melting and solidification are controlled such that theintegrity and dispersion of nanoparticles are preserved.

In some embodiments, the nanoparticles do not melt and do notsignificantly disperse from the original dispositions, relative to eachother, following melting of the metal matrix and then duringsolidification. In certain embodiments, the nanoparticles melt, soften(such as to become a glass), or form a liquid-solution solution, yet donot significantly disperse from the original dispositions, relative toeach other, following melting of the metal matrix and/or duringsolidification. When such nanoparticles resolidify (or undergo a phasetransition) during solidification of the melt, they assume theiroriginal dispositions or approximate coordinates thereof. In someembodiments, whether or not the nanoparticles melt, the nanoparticlesend up in a three-dimensional architecture in which the locations ofnanoparticles are different than the original dispositions, but may becorrelated and therefore predictable based on the startingfunctionalized feedstock.

In some embodiments, the composition is an ingot for producing a metalmatrix nanocomposite. In other embodiments, the composition itself is ametal matrix nanocomposite.

The microparticles may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof,for example. The nanoparticles may contain a compound selected from thegroup consisting of metals, ceramics, cermets, intermetallic alloys,oxides, carbides, nitrides, borides, polymers, carbon, and combinationsthereof, for example. In certain embodiments, the microparticles containAl, Si, and Mg (e.g., alloy AlSi10Mg), and the nanoparticles containtungsten carbide (WC).

Some variations of the invention provide a method of making a metalmatrix nanocomposite, the method comprising:

(a) providing a precursor composition comprising metal-containingmicroparticles and nanoparticles, wherein the nanoparticles arechemically and/or physically disposed on surfaces of the microparticles;

(b) consolidating the precursor composition into an intermediatecomposition comprising the metal-containing microparticles and thenanoparticles, wherein the nanoparticles are consolidated in athree-dimensional architecture throughout the intermediate composition;and

(c) processing the intermediate composition to convert the intermediatecomposition into a metal matrix nanocomposite.

In some embodiments, the precursor composition is in powder form. Insome embodiments, the intermediate composition is in ingot form.

The microparticles may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.The nanoparticles may contain a compound selected from the groupconsisting of metals, ceramics, cermets, intermetallic alloys, oxides,carbides, nitrides, borides, polymers, carbon, and combinations thereof.Typically, the compositions of the microparticles and nanoparticles aredifferent, although it is possible for the chemical composition to bethe same or similar while there are differences in physical properties(particle size, phases, etc.).

The composition may contain from about 10 wt % to about 99.9 wt % ofmicroparticles. In these or other embodiments, the composition containsfrom about 0.1 wt % to about 10 wt % of nanoparticles. Higherconcentrations of nanoparticles are possible, particularly when regionswith lower concentration are physically removed (as discussed later). Ametal matrix nanocomposite may be identified as a “cermet” when metalcontent is low, such as 20 wt % or less.

In some embodiments, at least 1% of the surface area of themicroparticles contains nanoparticles that are chemically and/orphysically disposed on the microparticle surfaces. When highernanoparticle concentrations are desired in the final material, it ispreferred that a higher surface area of the microparticles containsnanoparticles. In various embodiments, at least 1%, 2%, 5%, 10%, 20%,30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95% of the total surface area ofthe microparticles contains nanoparticles that are chemically and/orphysically disposed on the microparticle surfaces.

In some embodiments, the microparticles have an average microparticlesize from about 1 micron to about 1 centimeter. In various embodiments,the average microparticle size is about 5 microns, 10 microns, 50microns, 100 microns, 200 microns, 500 microns, 1 millimeter, 5millimeters, or 10 millimeters.

In some embodiments, the nanoparticles have an average nanoparticle sizefrom about 1 nanometer to about 1000 nanometers. In various embodiments,the average nanoparticle size is about 2, 5, 10, 25, 50, 100, 200, 300,400, 500, 600, 700, 800, or 900 nanometers.

In some embodiments, the metal matrix has a density from about 2 g/cm³to about 10 g/cm³. In some embodiments, the nanoparticles independentlyhave a density from about 1 g/cm³ to about 20 g/cm³.

“Consolidating” and “consolidation” refer to the conversion of aprecursor composition (e.g., feedstock powder) into an intermediatecomposition comprising the metal-containing microparticles and thenanoparticles. In various embodiments, consolidating in step (b)includes pressing, binding, sintering, or a combination thereof.Consolidating may alternatively or additionally include metal injectionmolding, extruding, isostatic pressing, powder forging, spray forming,metal additive manufacturing, and/or other known techniques. Theintermediate composition produced by step (b) may be referred to as agreen body.

In various embodiments, processing in step (c) includes pressing,sintering, mixing, dispersing, friction stir welding, extrusion, binding(such as with a polymer binder), melting, semi-solid melting, sintering,casting, or a combination thereof. Melting may include inductionmelting, resistive melting, skull melting, arc melting, laser melting,electron beam melting, semi-solid melting, or other types of melting(including convention and non-conventional melt processing techniques).Casting may include centrifugal, pour, or gravity casting, for example.Sintering may include spark discharge, capacitive-discharge, resistive,or furnace sintering, for example. Mixing may include convection,diffusion, shear mixing, or ultrasonic mixing, for example.

Steps (b) and (c) collectively convert the precursor composition (e.g.,the functionalized powder) into a green body or a finished body whichmay then be used for additional post processing, machined to a part, orother uses.

In some embodiments, the metal-matrix phase and the first reinforcementphase are each dispersed throughout the nanocomposite. In these or otherembodiments, the metal-matrix phase and the first reinforcement phaseare disposed in a layered configuration within the nanocomposite,wherein the layered configuration includes at least a first layercomprising the nanoparticles and at least a second layer comprising themetal-matrix phase.

The final metal matrix nanocomposite may have a cast microstructure, insome embodiments. By a “cast microstructure” it is meant that the metalmatrix nanocomposite is characterized by a plurality of dendrites andgrain boundaries within the microstructure. In some embodiments, thereis also a plurality of voids, but preferably no cracks or large phaseboundaries. A dendrite is a characteristic tree-like structure ofcrystals produced by faster growth of crystals along energeticallyfavorable crystallographic directions as molten metal freezes.

Note that while casting is a metal processing technique, a castmicrostructure is a structural feature, not necessarily tied to anyparticular process to make the microstructure. A cast microstructure cancertainly result from freezing (solidification) of molten metal or metalalloy. However, metal solidification can result in othermicrostructures, and cast microstructures can arise from othermetal-forming techniques. Metal processes that do not rely at all onmelting and solidification (e.g., forming processes) will not tend toproduce a cast microstructure.

A cast microstructure can generally be characterized by primary dendritespacing, secondary dendrite spacing, dendritic chemical segregationprofile, grain size, shrinkage porosity (if any), percent of secondaryphases, composition of secondary phases, and dendritic/equiaxedtransition, for example.

In some embodiments of the present invention, a cast microstructure isfurther characterized by an equiaxed, fine-grained microstructure.“Equiaxed” grains means that the grains are roughly equal in length,width, and height. Equiaxed grains can result when there are manynucleation sites arising from the plurality of nanoparticles containedon surfaces of microparticles, in the functionalized metal feedstock andtherefore in the final metal matrix nanocomposite.

In some embodiments of the present invention, a cast microstructure isfurther characterized by a dispersed microstructure. A dispersedmicrostructure generally arises from the large number of dendrites andgrain boundaries within the microstructure, which in turn arise from thelarge number of nanoparticles on surfaces of microparticles. The degreeof dispersion may be characterized by a dispersion length scale,calculated as the average spacing between nanoparticles and/or theaverage length scale in the metal phase between nanoparticles. Invarious embodiments, the dispersion length scale is from about 1nanometer to about 100 microns, such as from about 10 nanometers toabout 10 microns, or about 100 nanometers to about 1 micron.

Optionally, porosity may be removed or reduced in a cast microstructure.For example, a secondary heat and/or pressure (or other mechanicalforce) treatment may be done to minimize porous voids present in themetal matrix nanocomposite. Also, pores may be removed from the metalmatrix nanocomposite by physically removing (e.g., cutting away) aregion into which porous voids have segregated, such as viadensity-driven phase segregation. See FIGS. 10 and 11 for an example ofthis, in which voids present in the microstructure of FIG. 10 areremoved to arrive at the dispersed microstructure of FIG. 11. Thedispersion length scale in FIG. 11 is about 1-5 microns.

In addition to removal of voids, other post-working may be carried out,potentially resulting in other final microstructures that are not castmicrostructures, or that contain a mixture of microstructures. Forexample, forging can refine defects from cast ingots or continuous castbar, and can introduce additional directional strength, if desired.Preworking (e.g., strain hardening) can be done such as to produce agrain flow oriented in directions requiring maximum strength. The finalmicrostructure therefore may be a forged microstructure, or a mixedcast/forged microstructure, in certain embodiments. In variousembodiments, the metal matrix microstructure, on a volume basis, is atleast 10%, 25%, 50%, 75%, 90%, 95%, 99%, or 100% cast microstructure.

It is noted that friction stir processing requires rapid quenching toavoid settling and agglomeration that would occur during slowsolidification. Rapid quenching tends to produce microstructures thatare not cast microstructures as defined herein. Also, Bridgeman-typeconsolidation would be expected to present a microstructure that is nota dispersed cast microstructure.

Some variations of the present invention provide a raw material producedby a consolidation method of functionalized powder, to produce an ingotwhich may be used to make a nanocomposite, or is itself a nanocomposite.The metal alloys and nanoparticle compositions may vary widely, asdescribed elsewhere. Metal matrix nanocomposites herein may befabricated via compositional-bias assembly, density-bias assembly,hierarchical-size assembly, or other types of assembly of nanoparticles.The nanoparticles may stay the same composition upon ingot formation,the nanoparticles may react in some way to form a more favorablematerial for the nanocomposite, multiple different nanoparticles may beused, or any combination of this could occur.

Some graphical representations are shown in FIGS. 1 to 4, which areexemplary embodiments of metal matrix nanocomposites.

FIG. 1 depicts some embodiments in which a functionalized powdercontaining metal microparticles 105 coated with nanoparticles 110 isconsolidated into an ingot (or other material), such as by applicationof heat and pressure, containing nanoparticles 120 distributedthroughout a metal phase 115. The ingot 115/120 maintains athree-dimensional architecture of nanoparticles 120 uniformlydistributed throughout the metal matrix 115. As shown in the zoomed-inportion of the ingot (right-hand side of FIG. 1), the nanoparticles 120are oriented in a three-dimensional structure within the metal matrix115. In some embodiments, the three-dimensional structure is predictablebased on the starting material (i.e. the functionalized powdercontaining metal microparticles 105 coated with nanoparticles 110). Thatis, the dimensions of microparticles 105 and nanoparticles 110, and thespacing between individual microparticles 105 as well as betweenindividual nanoparticles 110, can be correlated to the spacing (in threedimensions) between individual nanoparticles 110 within the metal phase115 in the ingot.

FIG. 2 depicts some embodiments in which a functionalized powdercontaining metal microparticles 205 coated with nanoparticles 210 isconverted to a melt or ingot (or other material) containingnanoparticles 210 distributed throughout a metal phase 215. Thenanoparticles 210 then react in the melt to form a new distributed phase225 containing nanoparticles 220. The initial nanoparticles 210 haveundergone a chemical transformation via reaction, with the metal phase215, to form nanoparticles 220.

FIG. 3 depicts some embodiments starting with a functionalized powdercontaining metal microparticles 305 coated with nanoparticles 310 and320, which are different chemically and/or physically. Heat is appliedand the functionalized powder is converted to a melt or ingot (or othermaterial) containing nanoparticles 310 and 320 distributed in metalphase 315. The concentration of nanoparticles 310 and 320 may be uniformor non-uniform.

FIG. 4 depicts some embodiments starting with a functionalized powdercontaining metal microparticles 405 coated with nanoparticles 410 and420, which are different chemically and/or physically. Heat is appliedand the functionalized powder is converted to an ingot (or othermaterial) containing nanoparticles 410 and 420 distributed in metalphase 415. Then heat and/or pressure are applied and nanoparticles 420react to become nanoparticles 440 in a new phase, while nanoparticles410 do not react and are distributed as nanoparticles 410 in the metalphase 425.

FIG. 4 also illustrates that reinforcement phases may be created byin-situ chemical reactions with matrix constituents, instead of (or inaddition to) ex-situ methods. In ex-situ methods, reinforcements aresynthesized externally and then added into the matrix during compositefabrication.

Functionally Graded Metal Matrix Nanocomposites

This invention in some variations provides a functionally graded metalmatrix nanocomposite and a method for its fabrication. As intendedherein, a “functionally graded metal matrix nanocomposite” is a metalmatrix nanocomposite that exhibits a spatial gradient of one or moreproperties, derived from some spatial variation, within the metalmatrix, of a nanoparticle or nanoparticle phase. The property thatvaries may be mechanical, thermal, electrical, photonic, magnetic, orany other type of functional property. Some variations provide afunctionally graded metal matrix nanocomposite produced by adensity-driven separation (concentration or depletion) of thereinforcing particulate.

Metal matrix composites are typically fabricated with a micrometer-sizereinforcing particulate homogeneously dispersed in a metal matrix. Inorder to achieve larger amounts of strengthening, reducing the size ofthe reinforcement particulate to the nanoscale is preferred. However,reinforcement phase reactivity and inability to completely disperse hardphases at the nanoscale in melt processing limit productionopportunities of metal matrix nanocomposites.

Functionally graded metal matrix nanocomposite are conventionally evenmore difficult to process and are limited to friction stir processingwhich is geometrically and compositionally limited. Using metalfeedstock with nanoparticle functionalization as a means of mitigatingreactivity and dispersion issues in melt processing, functionally gradedmetal matrix nanocomposites can be produced with geometrically complexshapes and a broad spectrum of compositions. Known melt-processingtechniques such as centrifugal casting, gravity casting, orelectromagnetic separation casting may be employed to fabricate thefunctionally graded metal matrix nanocomposites.

Melt processing of metal matrix nanocomposites has traditionally provento be difficult in part due to particulate instability in the moltenmatrix and an inability to fully disperse the nanoparticles due tosurface energies. By contrast, in some embodiments of the presentinvention, reaction times in the liquid are reduced by utilizing apre-dispersed metal matrix nanocomposite feedstock powder, whereinnanoparticles are consolidated in a three-dimensional architecturethroughout the feedstock powder.

Density-driven phase separation may then be carried out to selectivelysegregate a first phase comprising the metal matrix and a second phasecomprising the nanoparticles. The segregation of the nanoparticles andthe metal matrix is useful because the nanoparticles are thenselectively contained in a solid reinforcement phase that has enhancedproperties compared to the metal matrix phase. The density-driven phaseseparation may result in a higher concentration or a lower concentration(i.e., depletion) of nanoparticles in any particular phase. The firstphase may be in liquid form or a liquid-solid solution, while thenanoparticles typically remain solid or at least as a distinct materialphase in the melt. Subsequent solidification of the melt produces agraded density of nanoparticles within the solid metal matrixnanocomposite.

Various forces may be employed to segregate nanoparticles by density,such as centrifugal, gravitational, thermal, electrical, acoustic, orother forces. Density-driven segregation may be accelerated by theapplication of an external force. Notably, density-driven phaseseparation can be used with metals that are not compatible with frictionstir processing.

The nanoparticle concentration may vary in volume fraction across thebulk of the material from 0 to 1.0, such as about 0.05, 0.1, 0.2, 0.3,0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 0.95. The local nanoparticleconcentrations (volume fractions) will depend on the starting amount ofnanoparticles (on microparticle surfaces), the properties of the metalmatrix, and the segregation technique employed. Following segregation,the region enriched in nanoparticles may have a volume fraction up to1.0, i.e. only nanoparticles in that phase. Similarly, the regiondepleted in nanoparticles may have a volume fraction of 0, i.e. nonanoparticles in that phase. The transition between low and highnanoparticle concentrations may be a gradual gradient (e.g., FIG. 5) ora sharp gradient (e.g., FIG. 12).

In addition to gradients in concentration, there can also be gradientsin particle sizes and material phases present, for example. Whendensity-driven segregation is used, there will of course also be adensity gradient. The difference between nanoparticle density and metalmatrix density may be at least 0.1, 0.5, 1, 2, 5, 10, or 15 g/cm³, forexample. The difference is about 13 g/cm³ in Example 1.

When density-driven segregation is used, depending on the densitydifferences, various length scales of gradients are possible. Forexample when the density difference is very large, nanoparticles mayform a high concentration in one region or layer of the material. Thegradient may be present over a length scale from about 10 microns toabout 1 centimeter or more, for example. In preferred embodiments, thegradient length scale is at least 100 microns.

Nanocomposites are often strong but may sometimes lack toughness, whichcan be problematic at high nanoparticle loading. By incorporatingfunctional grading, the material properties such as toughness can bemaintained while providing enhanced surface properties, enhanced bulkproperties, or enhanced overall properties. For example, a functionallygraded metal matrix nanocomposite may be designed to have high-hardnesssurfaces which improve wear characteristics, in comparison to metalmatrix composites reinforced with micrometer reinforcement. The improvedwear characteristics arise from the enhanced strengthening mechanismsintroduced at the nanoscale, as a result of a higher concentration ofnanoparticles at or near the surface.

In some embodiments, an ingot is made or obtained, for later producing ametal matrix nanocomposite. An “ingot” or equivalently “pre-dispersedingot” means a raw material that contains both a metal component and apre-dispersed reinforcing nanoparticle component. An ingot may beobtained after processing of a functionalized powder, or afterprocessing of a metal matrix nanocomposite. In some embodiments, theingot already contains a functional gradient of nanoparticle density. Insome embodiments, the ingot has or contains a microstructure indicativeof a material which consisted of powder precursors with nanoparticlesurface functionalization. This will result in a 3D network ofnanoparticles in the ingot.

An ingot may be a green body or a finished body. Ingot relativedensities may range from 10% to 100%, for example, calculated as apercentage of the theoretical density (void-free) of the componentscontained in the ingot.

The use of the ingot may vary. Further processing may result in theredistribution of nanoparticles throughout the structure. The ingot maybe processed in such a way that it has the distinct advantage ofcontaining a targeted volume fraction of nanoparticles determined duringfunctionalization and a uniform distribution due to the discretenanoparticle assembly on the surface of the metal-containingmicroparticles.

Some variations of the invention provide a functionally graded metalmatrix nanocomposite comprising a metal-matrix phase and a firstreinforcement phase containing first nanoparticles, wherein thenanocomposite contains a gradient in concentration of the firstnanoparticles through at least one dimension of the nanocomposite. Thegradient in concentration of the nanoparticles may be present in thenanocomposite over a length scale of at least 100 microns. Thenanocomposite has a cast microstructure, in some embodiments.

The metal-matrix phase may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.The first nanoparticles may contain a compound selected from the groupconsisting of metals, ceramics, cermets, intermetallic alloys, oxides,carbides, nitrides, borides, polymers, carbon, and combinations thereof.In some embodiments, the metal-matrix phase contains Al, Si, and Mg, andthe first nanoparticles contain tungsten carbide (WC).

The first nanoparticles may have an average particle size from about 1nanometer to about 1000 nanometers, such as about 10, 50, 100, 200, 300,400, 500, 600, 700, 800, or 900 nanometers. Some or all of the firstnanoparticles may be agglomerated such that the effective particle sizein the nanoparticle phase is larger than 1000 nanometers, in someembodiments.

The nanocomposite may contain from about 10 wt % to about 99.9 wt % ofthe metal-matrix phase, such as about 20, 30, 40, 50, 60, 70, 80, or 90wt %, for example.

The nanocomposite may contain from about 0.1 wt % to about 50 wt % ofthe first nanoparticles, such as about 1, 5, 10, 20, 30, or 40 wt %, forexample.

In some embodiments, the nanocomposite further comprises secondnanoparticles in the first reinforcement phase and/or in a secondreinforcement phase.

In some embodiments, the metal-matrix phase and the first reinforcementphase are each dispersed throughout the nanocomposite. In these or otherembodiments, the metal-matrix phase and the first reinforcement phaseare disposed in a layered configuration within the nanocomposite,wherein the layered configuration includes at least a first layercomprising the first nanoparticles and at least a second layercomprising the metal-matrix phase.

The nanocomposite may be present in an object or article that has atleast one dimension of 100 microns or greater, such as 200 microns, 500microns, 1 millimeter, 5 millimeters, 1 centimeter, or greater. Objector article sizes vary widely.

Certain variations of the invention provide a functionally graded metalmatrix nanocomposite comprising a metal-matrix phase containing Al, Si,and Mg and a reinforcement phase containing W and C, wherein thenanocomposite contains a gradient in concentration of the reinforcementphase through at least one dimension of the nanocomposite. Thenanocomposite may have a cast microstructure.

The metal-matrix phase contains aluminum alloy AlSi10Mg, in certainembodiments. AlSi10Mg is a typical casting alloy with good castingproperties and is often used for cast parts with thin walls and complexgeometry. It offers good strength, hardness, and dynamic properties andis therefore also used for parts subject to high loads. Adding areinforcement phase to AlSi10Mg offers additional benefits toproperties. The reinforcement phase contains tungsten carbide (WC), incertain embodiments.

In some embodiments, the metal-matrix phase and the reinforcement phaseare disposed in a layered configuration within the nanocomposite,wherein the layered configuration includes a first layer comprising W,C, Al, Si, and Mg, and a second layer comprising Al, Si, and Mg—that is,the first layer is enriched in W and C, such as in the form of WCnanoparticles.

In some embodiments, the nanocomposite is a master alloy, as furtherdiscussed below.

Other variations provide a method of making a functionally graded metalmatrix nanocomposite, the method comprising:

(a) providing a precursor composition (e.g., powder) comprisingmetal-containing microparticles and nanoparticles, wherein thenanoparticles are chemically and/or physically disposed on surfaces ofthe microparticles;

(b) consolidating the precursor composition into an intermediatecomposition (e.g., ingot) comprising the metal-containing microparticlesand the nanoparticles, wherein the nanoparticles are consolidated in athree-dimensional architecture throughout the intermediate composition;

(c) melting the intermediate composition to form a melt, wherein themelt segregates into a first phase comprising the metal-containingmicroparticles and a second phase comprising, or obtained from, thenanoparticles; and

(d) solidifying the melt to obtain a metal matrix nanocomposite with agradient in concentration of the nanoparticles through at least onedimension of the nanocomposite.

The microparticles may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.The nanoparticles may contain a compound selected from the groupconsisting of metals, ceramics, cermets, intermetallic alloys, oxides,carbides, nitrides, borides, polymers, carbon, and combinations thereof.In some embodiments, the microparticles contain Al, Si, and Mg, and thenanoparticles contain tungsten carbide (WC).

In various embodiments, step (b) includes pressing, binding, sintering,or a combination thereof.

In various embodiments, step (c) includes pressing, sintering, mixing,dispersing, friction stir welding, extrusion, binding, melting,semi-solid melting, capacitive discharge sintering, casting, or acombination thereof. Step (c) may also include holding the melt for aneffective dwell time to cause density-driven segregation of the firstphase from the second phase. The dwell time may be selected from about 1minute to about 8 hours, for example. In some embodiments, step (c)includes exposing the melt to an external force selected fromgravitational, centrifugal, mechanical, electromagnetic, or acombination thereof.

Step (d) may include directional solidification or progressivesolidification of the melt. Directional solidification and progressivesolidification are types of solidification within castings. Directionalsolidification is solidification that occurs from the farthest end ofthe casting and works its way towards the passage through which liquidmaterial is introduced into a mold. Progressive solidification issolidification that starts at the walls of the casting and progressesperpendicularly from that surface.

The metal-matrix phase and the reinforcement phase may be each dispersedthroughout the nanocomposite. In these or other embodiments, themetal-matrix phase and the reinforcement phase are disposed in a layeredconfiguration within the nanocomposite, wherein the layeredconfiguration includes at least a first layer comprising thenanoparticles and at least a second layer comprising the metal-matrixphase. The nanoparticles may undergo some amount of agglomeration.Agglomeration between nanoparticles may result in nanoparticles beingchemically or physically bound together. Individual nanoparticles may ormay not be present or detectable in the reinforcement phase, and thelength scale associated with the nanoparticles may become greater than1000 nm.

The gradient in concentration of the nanoparticles may be present in thenanocomposite over a length scale of at least 10 microns, such as atleast 100 microns, up to about 1 centimeter or more, for example.

In some embodiments, the functionally graded metal matrix nanocompositehas a cast microstructure, defined above. In certain embodiments, thereis a functional gradient in the microstructure itself, related to orindependent of the concentration gradient.

FIGS. 5 to 10 exhibit various embodiments of functionally graded metalmatrix nanocomposites.

FIG. 5 depicts some embodiments starting with nanoparticles 510predistributed in a metal matrix 505, such as in an ingot. The ingot maybe obtained from heating a functionalized powder containing metalmicroparticles coated with nanoparticles, as shown in FIGS. 1-4. Heat isapplied to the ingot which undergoes density-driven phase segregation inwhich nanoparticles 510 migrate toward the surface (against gravity) dueto a density less than the density of the molten matrix 515. Aftersolidification, the resulting functionally graded metal matrixnanocomposite contains a higher concentration of nanoparticles 510 at ornear the surface, compared to the bulk of the material, within the metalphase 525.

FIG. 6 depicts some embodiments starting with nanoparticles 610predistributed in a metal matrix 605, such as in an ingot. The ingot maybe obtained from heating a functionalized powder containing metalmicroparticles coated with nanoparticles, as shown in FIGS. 1-4. Heat isapplied to the ingot which undergoes density-driven phase segregation inwhich nanoparticles 610 migrate away from the surface (in the directionof gravity) due to a density greater than the density of the moltenmatrix 615. After solidification, the resulting functionally gradedmetal matrix nanocomposite contains a higher concentration ofnanoparticles 610 at or near the distal region away from the surface,compared to the bulk of the material, within the metal phase 625.

FIG. 7 depicts some embodiments starting with codispersed nanoparticles710 and 720 predistributed in a metal matrix 705, such as in an ingot.The ingot may be obtained from heating a functionalized powdercontaining metal microparticles coated with nanoparticles, as shown inFIGS. 1-4. Heat is applied to the ingot which undergoes density-drivenphase segregation in which nanoparticles 710 migrate away from thesurface (in the direction of gravity) due to a density greater than thedensity of the molten matrix 715, while nanoparticles 720 migrate towardthe surface (against gravity) due to a density less than the density ofthe molten matrix 715. After solidification, the resulting functionallygraded metal matrix nanocomposite contains a higher concentration ofnanoparticles 710 at or near the distal region away from the surface,and a higher concentration of nanoparticles 720 at or near the surface,compared to the bulk of the material, within the metal phase 725.

FIG. 8 depicts some embodiments starting with codispersed nanoparticles810 and 820 predistributed in a metal matrix 805, such as in an ingot.The ingot may be obtained from heating a functionalized powdercontaining metal microparticles coated with nanoparticles, as shown inFIGS. 1-4. Heat is applied to the ingot which undergoes density-drivenphase segregation in which nanoparticles 810 migrate away from thesurface (in the direction of gravity) due to a density greater than thedensity of the molten matrix 815. In this embodiment, nanoparticles 820also migrate away from the surface (in the direction of gravity) due toa density greater than the density of the molten matrix 815, but thedensity of nanoparticles 820 is less than the density of nanoparticles810. Therefore, nanoparticles 820 remain more dispersed within themolten metal matrix 815, compared to the nanoparticles 810. Aftersolidification, the resulting functionally graded metal matrixnanocomposite contains a higher concentration of both nanoparticles 810and 820 at or near the distal region away from the surface, compared tothe bulk of the material, within the metal phase 825. The gradients ofnanoparticles 810/820 concentrations are different.

FIG. 9 depicts some embodiments starting with codispersed nanoparticles910 and 920 predistributed in a metal matrix 905, such as in an ingot.The ingot may be obtained from heating a functionalized powdercontaining metal microparticles coated with nanoparticles, as shown inFIGS. 1-4. Heat is applied to the ingot which undergoes density-drivenphase segregation in which nanoparticles 910 migrate toward the surface(against gravity) due to a density less than the density of the moltenmatrix 915. In this embodiment, nanoparticles 920 also migrate towardthe surface due to a density less than the density of the molten matrix915, but the density of nanoparticles 920 is greater than the density ofnanoparticles 930. Therefore, nanoparticles 920 are more dispersedwithin the molten metal matrix 915, compared to the nanoparticles 910.After solidification, the resulting functionally graded metal matrixnanocomposite contains a higher concentration of both nanoparticles 910and 920 at or near the surface, compared to the bulk of the material,within the metal phase 925. The gradients of nanoparticles 910/920concentrations are different.

FIG. 10 is an SEM image of a cross-section (side view) of an exemplaryAlSi10Mg—WC functionally graded metal matrix nanocomposite, according toExample 1 (described in the EXAMPLES below).

Master Alloy Metal Matrix Nanocomposites

A “master alloy” is well-defined in the art and refers to a concentratedalloy source which can be added to a metal being processed, to introducethe appropriate alloying elements into the system. Master alloys areparticularly useful when the alloying elements are difficult to disperseor in low weight quantities. In the case of the dispersion difficulties,pre-dispersed master alloys increase wetting and avoid agglomeration. Inthe case of low quantities, it is much easier to control additions whenheavier weights of pre-alloyed material can be added, to avoid weighingerrors for the minor alloying elements.

A “master alloy metal matrix nanocomposite” or equivalently “masteralloy nanocomposite” herein means a metal matrix nanocomposite withgreater than 0.1 wt % nanoparticles distributed in a metal or metalalloy matrix, suitable for further processing through a variety ofdifferent routes (melt processing, machining, forging, etc.) into afinal product. The concentration of nanoparticles is typically at least1 wt %.

In some variations of the invention, a functionally graded metal matrixnanocomposite is fabricated, followed by removal of one or more phasesnot containing nanoparticles from the nanocomposite, to generate amaster alloy metal matrix nanocomposite.

The production of a master alloy metal matrix nanocomposite allows for ahigh volume loading of reinforcement phases into metal matrices. Byconsolidating a homogenously dispersed nanoparticle reinforcement phase,such as via density-driven phase separation, and then removing a portionthat does not contain the nanoparticle reinforcement phase, a masteralloy is obtained. The master alloy may be used in further processing toproduce a final geometrical configuration, such as in melt processingand casting.

These methods provide low-cost, high-volume production of master alloymetal matrix nanocomposites with high volume loading of nanoparticulatereinforcement. Reaction times may be minimized by using a pre-dispersedmetal matrix nanocomposite feedstock powder or feedstock ingot.

Some variations of the invention provide a method of making a masteralloy metal matrix nanocomposite, the method comprising:

(a) providing an ingot composition comprising metal-containingmicroparticles and nanoparticles, wherein the nanoparticles arechemically and/or physically disposed on surfaces of the microparticles,and wherein the nanoparticles are consolidated in a three-dimensionalarchitecture throughout the ingot composition;

(b) melting the ingot composition to form a melt, wherein the meltsegregates into a first phase comprising the metal-containingmicroparticles and a second phase comprising the nanoparticles;

(c) solidifying the melt to obtain a metal matrix nanocomposite with agradient in concentration of the nanoparticles through at least onedimension of the nanocomposite; and

(d) removing a fraction of the metal matrix nanocomposite containing alower concentration of the nanoparticles compared to the remainder ofthe metal matrix nanocomposite, thereby producing a master alloy metalmatrix nanocomposite.

The microparticles may contain an element selected from the groupconsisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.The nanoparticles may contain a compound selected from the groupconsisting of metals, ceramics, cermets, intermetallic alloys, oxides,carbides, nitrides, borides, polymers, carbon, and combinations thereof.In certain embodiments, the microparticles contain Al, Si, and Mg, andthe nanoparticles contain tungsten carbide (WC).

The processing in steps (b) and (c) takes a pre-dispersed ingot, orother starting ingot composition, as a raw material and produces afunctionally graded metal matrix nanocomposite.

Step (b) may further include pressing, sintering, mixing, dispersing,friction stir welding, extrusion, binding, capacitive dischargesintering, casting, or a combination thereof. Step (b) may includeholding the melt for an effective dwell time (e.g., about 1 minute to 8hours) to cause density-driven segregation of the first phase from thesecond phase. Optionally, step (b) may include exposing the melt to anexternal force selected from gravitational, centrifugal, mechanical,electromagnetic, or a combination thereof.

Step (c) may include directional solidification or progressivesolidification of the melt, if desired. Directional solidification issolidification that occurs from the farthest end of the casting andworks its way towards the passage through which liquid material isintroduced into a mold. Progressive solidification is solidificationthat starts at the walls of the casting and progresses perpendicularlyfrom that surface.

The gradient in concentration of the first nanoparticles may be presentin the metal matrix nanocomposite over a length scale of at least 100microns.

In some embodiments, the metal-matrix phase and the first reinforcementphase are each dispersed throughout the metal matrix nanocomposite. Inthese or other embodiments, the metal-matrix phase and the firstreinforcement phase are disposed in a layered configuration within themetal matrix nanocomposite, wherein the layered configuration includesat least a first layer comprising the nanoparticles and at least asecond layer comprising the metal-matrix phase.

Step (d) may include includes machining, ablation, reaction,dissolution, evaporation, selective melting, or a combination thereof.In certain embodiments, step (d) provides two distinct master alloymetal matrix nanocomposites. A number of heating methods and dwell timesare appropriate for the production of density-driven master alloy metalmatrix nanocomposites.

In some embodiments, a method of fabrication of a master alloy metalmatrix nanocomposite starts by using a pre-dispersed ingot as a rawmaterial with a metal component and a reinforcing particulate. Thisingot is taken to a liquid or a semi-solid phase through processing,wherein the metal component enters a molten liquid or semi-solid phasewith a dispersed reinforcing component (nanoparticles).

The reinforcing component segregates through density-driven segregation,in some embodiments. In particular, the matrix is solidified and thereinforcing component is separated by density into one or morehigher-volume fractions (compared to the matrix). The low-volumefraction component of the whole solid is then removed, at leastpartially, to leave behind a final product of a high-volume fractionmaster alloy metal matrix nanocomposite.

Compositions of this master alloy vary widely, according to selection ofthe matrix metal(s) or metal alloy(s) in combination with nanoparticlesof arbitrary composition, including other metals or metal alloys.Reinforcing nanoparticles are preferably less than 1000 nm in size, morepreferably less than 250 nm, with any geometrical configuration (rod,sphere, prism, etc.). Note that the removed low-density material may berecycled and used in subsequent processing. By producing a master alloywhich may be added to a targeted alloy system in the molten state, fullydispersed metal matrix nanocomposites may be created and later processedunder conventional, cost-effective pyro-metallurgy approaches.

In some embodiments, the metal matrix nanocomposite in step (c) ischaracterized by a cast microstructure. The final master alloy metalmatrix nanocomposite(s) may have a cast microstructure. A castmicrostructure is characterized in that it includes a plurality ofdendrites (from crystal growth) and grain boundaries within themicrostructure. In some embodiments, there is also a plurality of voids,but preferably no cracks or large phase boundaries.

In some embodiments, a cast microstructure is further characterized byan equiaxed, fine-grained microstructure. Equiaxed grains are roughlyequal in length, width, and height. Equiaxed grains can result whenthere are many nucleation sites arising from the plurality ofnanoparticles contained on surfaces of microparticles, in thefunctionalized metal feedstock and therefore in the master alloy metalmatrix nanocomposite.

In some embodiments, a cast microstructure is further characterized by adispersed microstructure. A dispersed microstructure generally arisesfrom the large number of dendrites and grain boundaries within themicrostructure, which in turn arise from the large number ofnanoparticles initially on surfaces of microparticles. The degree ofdispersion may be characterized by a dispersion length scale, calculatedas the average spacing between nanoparticles and/or the average lengthscale in the metal phase between nanoparticles. In various embodiments,the dispersion length scale is from about 1 nanometer to about 100microns, such as from about 10 nanometers to about 10 microns, or about100 nanometers to about 1 micron.

Optionally, porosity may be removed or reduced in a cast microstructure.For example, a secondary heat and/or pressure (or other mechanicalforce) treatment may be done to minimize porous voids present in themetal matrix nanocomposite. Also, pores may be removed from the metalmatrix nanocomposite by physically removing (e.g., cutting away) aregion into which porous voids have segregated, such as viadensity-driven phase segregation. The desired master alloy may havefewer voids, or no voids, compared to the region removed.

In addition to removal of voids, other post-working may be carried out,potentially resulting in other final microstructures that are not castmicrostructures, or that contain a mixture of microstructures. Forexample, forging can refine defects from cast ingots or continuous castbar, and can introduce additional directional strength, if desired.Preworking (e.g., strain hardening) can be done such as to produce agrain flow oriented in directions requiring maximum strength. The masteralloy microstructure therefore may be a forged microstructure, or amixed cast/forged microstructure, in certain embodiments. In variousembodiments, the master alloy metal matrix microstructure, on a volumebasis, is at least 10%, 25%, 50%, 75%, 90%, 95%, 99%, or 100% castmicrostructure.

The master alloy may ultimately be processed in various parts. Theseparts may be produced by a variety of processes, and therefore a finalpart may or may not have a cast microstructure. Metal-part formingoperations include, but are not limited to, forging, rolling, extrusion,drawing, sand casting, die casting, investment casting, powdermetallurgy, welding, additive manufacturing, or others. A castmicrostructure may be desired in the final part, or a differentmicrostructure may be desired, such as a forged microstructure. A castmicrostructure for the master alloy may be preferred for the performanceand quality of a final part, in some embodiments.

FIGS. 11 to 15 exhibit several, non-limiting embodiments of master alloymetal matrix nanocomposites.

FIG. 11 is an SEM image of a cross-section (side view) of an exemplaryAlSi10Mg—WC master alloy metal matrix nanocomposite, according toExample 2 (described in the EXAMPLES below).

FIG. 12 depicts some embodiments starting with nanoparticles 1210predistributed in a metal matrix 1205, such as in an ingot. Heat isapplied to the ingot undergoes density-driven phase segregation in whichnanoparticles 1210 migrate toward the surface (against gravity) due to adensity less than the density of the molten matrix 1215. Aftersolidification, the resulting functionally graded metal matrixnanocomposite contains a higher concentration of nanoparticles 1210 ator near the surface, compared to the bulk of the material, within themetal phase 1225. A portion of the solid 1225, with relatively lowerconcentration of nanoparticles 1210 (or no nanoparticles as in thisillustration), is then removed. The result is a master alloy metalmatrix nanocomposite enriched with nanoparticles 1210 in metal matrix1225.

FIG. 13 depicts some embodiments starting with nanoparticles 1310predistributed in a metal matrix 1305, such as in an ingot. Heat isapplied to the ingot which undergoes density-driven phase segregation inwhich nanoparticles 1310 migrate away from the surface (in the directionof gravity) due to a density greater than the density of the moltenmatrix 1315. After solidification, the resulting functionally gradedmetal matrix nanocomposite contains a higher concentration ofnanoparticles 1310 at or near the distal region away from the surface,compared to the bulk of the material, within the metal phase 1325. Aportion of the solid 1325, with relatively lower concentration ofnanoparticles 1310 (or no nanoparticles as in this illustration), isthen removed. The result is a master alloy metal matrix nanocompositeenriched with nanoparticles 1310 in metal matrix 1325.

FIG. 14 depicts some embodiments starting with codispersed nanoparticles1410 and 1420 predistributed in a metal matrix 1405, such as in aningot. Heat is applied to the ingot which undergoes density-driven phasesegregation in which nanoparticles 1410 migrate away from the surface(in the direction of gravity) due to a density greater than the densityof the molten matrix 1415. In this embodiment, nanoparticles 1420 alsomigrate away from the surface (in the direction of gravity) due to adensity greater than the density of the molten matrix 1415, but thedensity of nanoparticles 1420 is greater than the density ofnanoparticles 1410. After solidification, the resulting functionallygraded metal matrix nanocomposite contains a higher concentration ofboth nanoparticles 1410 and 1420 at or near the distal region away fromthe surface, compared to the bulk of the material, within the metalphase 1425. A portion of the solid 1425, with relatively lowerconcentration of nanoparticles 1410/1420 (or no nanoparticles as in thisillustration), is then removed. The result is a master alloy metalmatrix nanocomposite enriched with nanoparticles 1410 and 1420 in metalmatrix 1425. Note that the layered configuration in FIG. 14 is possiblebecause the densities of nanoparticles 1410 and 1420 are different. Inother embodiments, when the densities are the same or similar,nanoparticles 1410 and 1420 will tend to be uniformly dispersed withinthe final master alloy metal matrix nanocomposite.

FIG. 15 depicts some embodiments starting with codispersed nanoparticles1510 and 1520 predistributed in a metal matrix 1505, such as in aningot. Heat is applied to the ingot which undergoes density-driven phasesegregation in which nanoparticles 1510 migrate away from the surface(in the direction of gravity) due to a density greater than the densityof the molten matrix 1515, while nanoparticles 1520 migrate toward thesurface (against gravity) due to a density less than the density of themolten matrix 1515. After solidification, the resulting functionallygraded metal matrix nanocomposite contains a higher concentration ofnanoparticles 1510 at or near the distal region away from the surface,and a higher concentration of nanoparticles 1520 at or near the surface,compared to the bulk of the material, within the metal phase 1525. Aportion of the solid 1525, with relatively lower concentration ofnanoparticles 1510/1520 (or no nanoparticles as in this illustration),is then removed. Two distinct master alloy metal matrix nanocompositesare fabricated simultaneously. One master alloy metal matrixnanocomposite is enriched with nanoparticles 1510 in metal matrix 1525.The other master alloy metal matrix nanocomposite is enriched withnanoparticles 1520 in metal matrix 1525.

Functionalized Metal Feedstocks for Producing Metal MatrixNanocomposites

Powder materials are a general class of feedstock for a powdermetallurgy process, including but not limited to additive manufacturing,injection molding, and press and sintered applications. As intendedherein, “powder materials” refers to any powdered ceramic, metal,polymer, glass, or composite or combination thereof. In someembodiments, the powder materials are metals or metal-containingcompounds, but this disclosure should not be construed as limited tometal processing. Powder sizes are typically between about 1 micron andabout 1 mm, but in some cases could be as much as about 1 cm.

The powdered material may be in any form in which discrete particles canbe reasonably distinguished from the bulk. The powder materials are notalways observed as loose powders and may be present as a paste,suspension, or green body. A green body is an object whose mainconstituent is weakly bound powder material, before it has been meltedand solidified. For instance, a filler rod for welding may consist ofthe powder material compressed into a usable rod.

Particles may be solid, hollow, or a combination thereof. Particles canbe made by any means including, for example, gas atomization, milling,cryomilling, wire explosion, laser ablation, electrical-dischargemachining, or other techniques known in the art. The powder particlesmay be characterized by an average aspect ratio from about 1:1 to about100:1. The “aspect ratio” means the ratio of particle length to width,expressed as length:width. A perfect sphere has an aspect ratio of 1:1.For a particle of arbitrary geometry, the length is taken to be themaximum effective diameter and the width is taken to be the minimumeffective diameter.

In some embodiments, the particles are in the shape of rods. By “rod” itis meant a rod-shaped particle or domain shaped like long sticks,dowels, or needles. The average diameter of the rods may be selectedfrom about 5 nanometers to about 100 microns, for example. Rods need notbe perfect cylinders, i.e. the axis is not necessarily straight and thediameter is not necessarily a perfect circle. In the case ofgeometrically imperfect cylinders (i.e. not exactly a straight axis or around diameter), the aspect ratio is the actual axial length, along itsline of curvature, divided by the effective diameter, which is thediameter of a circle having the same area as the average cross-sectionalarea of the actual nanorod shape.

The powder material particles may be anisotropic. As meant herein,“anisotropic” particles have at least one chemical or physical propertythat is directionally dependent. When measured along different axes, ananisotropic particle will have some variation in a measurable property.The property may be physical (e.g., geometrical) or chemical in nature,or both. The property that varies along multiple axes may simply be thepresence of mass; for example, a perfect sphere would be geometricallyisotropic while a cylinder is geometrically anisotropic. The amount ofvariation of a chemical or physical property may be 5%, 10%, 20%, 30%,40%, 50%, 75%, 100% or more.

“Solidification” generally refers to the phase change from a liquid to asolid. In some embodiments, solidification refers to a phase changewithin the entirety of the powder volume. In other embodiments,solidification refers to a phase change at the surface of the particlesor within a fractional volume of the powder material. In variousembodiments, at least (by volume) 1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%,40%, 50%, 60%, 70%, 80%, 90%, 95%, 99%, or 100% of the powdered materialis melted to form the liquid state.

For a metal or mixtures of metals, solidification generally results inone or more solid metal phases that are typically crystalline, butsometimes amorphous. Ceramics also may undergo crystallinesolidification or amorphous solidification. Metals and ceramics may forman amorphous region coinciding with a crystalline region (e.g., insemicrystalline materials). In the case of certain polymers and glasses,solidification may not result in a crystalline solidification. In theevent of formation of an amorphous solid from a liquid, solidificationrefers to a transition of the liquid from above the glass-transitiontemperature to an amorphous solid at or below the glass-transitiontemperature. The glass-transition temperature is not alwayswell-defined, and sometimes is characterized by a range of temperatures.

“Surface functionalization” refers to a surface modification on thepowdered materials, which modification significantly affects thesolidification behavior (e.g., solidification rate, yield, selectivity,heat release, etc.) of the powder materials. In various embodiments, apowdered material is functionalized with about 1%, 2%, 5%, 10%, 15%,20%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, 85%, 90%, 95%, 99%, or 100% ofthe surface area of the powdered material having thesurface-functionalization modifications. The surface modification maybea surface-chemistry modification, a physical surface modification, or acombination thereof.

In some embodiments, the surface functionalization includes ananoparticle coating and/or a microparticle coating. The nanoparticlesand/or microparticles may include a metal, ceramic, polymer, or carbon,or a composite or combination thereof. The surface functionalization mayinclude a particle assembly that is chemically or physically disposed onthe surface of the powder materials.

Due to the small size of nanoparticles and their reactivity, thebenefits provided herein may be possible with less than 1% surface areacoverage. In the case of functionalization with a nanoparticle of thesame composition as the base powder, a surface-chemistry change may notbe detectible and can be characterized by topological differences on thesurface, for example. Functionalization with a nanoparticle of the samecomposition as the base powder may be useful to reduce the melting pointin order to initiate sintering at a lower temperature, for example.

In some embodiments, microparticles coat micropowders or macropowders.The micropowder or macropowder particles may include ceramic, metal,polymer, glass, or combinations thereof. The microparticles (coating)may include metal, ceramic, polymer, carbon, or combinations thereof. Inthe case of microparticles coating other micropowders or macropowders,functionalization preferably means that the coating particles are ofsignificantly different dimension(s) than the base powder. For example,the microparticles may be characterized by an average dimension (e.g.,diameter) that is less than 20%, 10%, 5%, 2%, or 1% of the largestdimension of the coated powders.

In some embodiments, surface functionalization is in the form of acontinuous coating or an intermittent coating. A continuous coatingcovers at least 90% of the surface, such as about 95%, 99%, or 100% ofthe surface (recognizing there may be defects, voids, or impurities atthe surface). An intermittent coating is non-continuous and covers lessthan 90%, such as about 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, 5%, 2%,1%, or less of the surface. An intermittent coating may be uniform(e.g., having a certain repeating pattern on the surface) or non-uniform(e.g., random).

In general, the coating may be continuous or discontinuous. The coatingmay have several characteristic features. In one embodiment, the coatingmay be smooth and conformal to the underlying surface. In anotherembodiment, the coating may be nodular. The nodular growth ischaracteristic of kinetic limitations of nucleation and growth. Forexample, the coating may look like cauliflower or a small fractalgrowing from the surface. These features can be affected by theunderling materials, the method of coating, reaction conditions, etc.

A coating may or may not be in the form of nanoparticles ormicroparticles. That is, the coating may be derived from nanoparticlesor microparticles, while discrete nanoparticles or microparticles may nolonger be present. Various coating techniques may be employed, such as(but not limited to) electroless deposition, immersion deposition, orsolution coating. The coating thickness is preferably less than about20% of the underlying particle diameter, such as less than 15%, 10%, 5%,2%, or 1% of the underlying particle diameter.

In some embodiments, the surface functionalization also includes directchemical or physical modification of the surface of the powdermaterials, such as to enhance the bonding of the nanoparticles ormicroparticles. Direct chemical modification of the surface of thepowder materials, such as addition of molecules, may also be utilized toaffect the solidification behavior of the powder materials. A pluralityof surface modifications described herein may be used simultaneously.

Nanoparticles are particles with the largest dimension between about 1nm and 1000 nm. A preferred size of nanoparticles is less than 250 nm,more preferably less than 100 nm. Microparticles are particles with thelargest dimension between about 1 micron and 1000 microns. Nanoparticlesor microparticles may be metal, ceramic, polymer, carbon-based, orcomposite particles, for example. The nanoparticle or microparticle sizemay be determined based on the desired properties and final function ofthe assembly.

Nanoparticles or microparticles may be spherical or of arbitrary shapewith the largest dimension typically not exceeding the above largestdimensions. An exception is structures with extremely high aspectratios, such as carbon nanotubes in which the dimensions may include upto 100 microns in length but less than 100 nm in diameter. Thenanoparticles or microparticles may include a coating of one or morelayers of a different material. Mixtures of nanoparticles andmicroparticles may be used. In some embodiments, microparticlesthemselves are coated with nanoparticles, and themicroparticle/nanoparticle composite is incorporated as a coating orlayer on the powder material particles.

Some variations provide a powdered material comprising a plurality ofparticles, wherein the particles are fabricated from a first material(e.g., ceramic, metal, polymer, glass, or combinations thereof), andwherein each of the particles has a particle surface area that issurface-functionalized (such as continuously or intermittently) withnanoparticles and/or microparticles selected to control solidificationof the powdered material from a liquid state to a solid state. Thenanoparticles and/or microparticles may include metal, ceramic, polymer,carbon, or combinations thereof.

In some embodiments, the powdered material is characterized in that onaverage at least 1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%,50%, or more of the particle surface area is surface-functionalized withthe nanoparticles and/or the microparticles.

In some embodiments, the nanoparticles and/or microparticles areselected to control solidification of a portion of the powderedmaterial, such as a region of powdered material for which solidificationcontrol is desired. Other regions containing conventional powderedmaterials, without nanoparticles and/or microparticles, may be present.In some embodiments, the nanoparticles and/or microparticles areselected to control solidification of a portion of each the particles(e.g., less than the entire volume of a particle, such as an outershell).

Various material combinations are possible. In some embodiments, thepowder particles are ceramic and the nanoparticles and/or microparticlesare ceramic. In some embodiments, the powder particles are ceramic andthe nanoparticles and/or microparticles are metallic. In someembodiments, the powder particles are polymeric and the nanoparticlesand/or microparticles are metallic, ceramic, or carbon-based. In someembodiments, the powder particles are glass and the nanoparticles and/ormicroparticles are metallic. In some embodiments, the powder particlesare glass and the nanoparticles and/or microparticles are ceramic. Insome embodiments, the powder particles are ceramic or glass and thenanoparticles and/or microparticles are polymeric or carbon-based, andso on.

Exemplary ceramic materials for the powders, or the nanoparticles and/ormicroparticles, include (but are not limited to) SiC, HfC, TaC, ZrC,NbC, WC, TiC, TiC_(0.7)N_(0.3), VC, B₄C, TiB₂, HfB₂, TaB₂, ZrB₂, WB₂,NbB₂, TaN, HfN, BN, ZrN, TiN, NbN, VN, Si₃N₄, Al₂O₃, MgAl₂O₃, HfO₂,ZrO₂, Ta₂O₅, TiO₂, SiO₂, and oxides of rare-earth elements Y, La, Ce,Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho Er, Tm, Yb, and/or Lu.

Exemplary metallic materials for the powders, or the nanoparticlesand/or microparticles, include (but are not limited to) Sc, Ti, V, Cr,Y, Zr, Nb, Mo, Ru, Rh, Pd, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho Er,Tm, Yb, Lu, Ta, W, Re, Os, Ir, Pt, Si, or B.

Exemplary polymer materials for the powders, or the nanoparticles and/ormicroparticles, include (but are not limited to) thermoplastic organicor inorganic polymers, or thermoset organic or inorganic polymers.Polymers may be natural or synthetic.

Exemplary glass materials for the powders include (but are not limitedto) silicate glasses, porcelains, glassy carbon, polymer thermoplastics,metallic alloys, ionic liquids in a glassy state, ionic melts, andmolecular liquids in a glassy state.

Exemplary carbon or carbon-based materials for the nanoparticles and/ormicroparticles include (but are not limited to) graphite, activatedcarbon, graphene, carbon fibers, carbon nanostructures (e.g., carbonnanotubes), and diamond (e.g., nanodiamonds).

These categories of materials are not mutually exclusive; for example agiven material may be metallic/ceramic, a ceramic glass, a polymericglass, etc.

The selection of the coating/powder composition will be dependent on thedesired properties and should be considered on a case-by-case basis.Someone skilled in the art of material science or metallurgy will beable to select the appropriate materials for the intended process, basedon the information provided in this disclosure. The processing and finalproduct configuration should also be dependent on the desiredproperties. Someone skilled in the art of material science, metallurgy,and/or mechanical engineering will be able to select the appropriateprocessing conditions for the desired outcome, based on the informationprovided in this disclosure.

In some embodiments, a method of controlling solidification of apowdered material comprises:

providing a powdered material comprising a plurality of particles,wherein the particles are fabricated from a first material, and whereineach of the particles has a particle surface area that issurface-functionalized with nanoparticles and/or microparticles;

melting at least a portion of the powdered material to a liquid state;and

semi-passively controlling solidification of the powdered material fromthe liquid state to a solid state.

As intended in this description, “semi-passive control,” “semi-passivelycontrolling,” and like terminology refer to control of solidificationduring heating, cooling, or both heating and cooling of thesurface-functionalized powder materials, wherein the solidificationcontrol is designed prior to melting through selected functionalizationand is not actively controlled externally once the melt-solidificationprocess has begun. Note that external interaction is not necessarilyavoided. In some embodiments, semi-passive control of solidificationfurther includes selecting the atmosphere (e.g., pressure, humidity, orgas composition), temperature, or thermal input or output. These factorsas well as other factors known to someone skilled in the art may or maynot be included in semi-passive control.

Exemplary semi-passive control processes, enabled through surfacefunctionalization as described herein, will now be illustrated.

One route to control nucleation is the introduction, into the liquidphase, of nanoparticles derived from a coating described above. Thenanoparticles may include any material composition described above andmay be selected based on their ability to wet into the melt. Upon meltinitiation, the nanoparticles wet into the melt pool as dispersedparticles which, upon cooling, serve as nucleation sites, therebyproducing a fine-grained structure with observable nucleation sites inthe cross-section. In some embodiments, the density of nucleation sitesis increased, which may increase the volumetric freezing rate due to thenumber of growing solidification fronts and the lack of a nucleationenergy barrier.

In an exemplary embodiment, ceramic nanoparticles, e.g. TiB₂ or Al₂O₃nanoparticles, are coated onto aluminum alloy microparticles. Theceramic nanoparticles are introduced into an aluminum alloy melt pool inan additive manufacturing process. The nanoparticles then disperse inthe melt pool and act as nucleation sites for the solid. The additionalwell-dispersed nucleation sites can mitigate shrinkage cracks (hotcracking). Shrinkage cracks typically occur when liquid cannot reachcertain regions due to blockage of narrow channels between solidifyinggrains. An increase in nucleation sites can prevent formation of long,narrow channels between solidifying grains, because multiple smallgrains are growing, instead of few large grains.

In another exemplary embodiment, nanoparticles act as nucleation sitesfor a secondary phase in an alloy. The nanoparticles may comprise thesecondary phase or a material that nucleates the secondary phase (due tosimilar crystal structures, for instance). This embodiment can bebeneficial if the secondary phase is responsible for blockinginterdendritic channels leading to hot cracking. By nucleating manysmall grains of the secondary phase, a large grain that might block thenarrow channel between the dendrites can be avoided. Furthermore, thisembodiment can be beneficial if the secondary phase tends to form acontinuous phase between the grains of the primary phase, which promotesstress corrosion cracking. By providing additional nucleation sites forthe secondary phase, this secondary phase may be broken up andinterdispersed, preventing it from forming a continuous phase betweengrains of the primary alloy. By breaking up a secondary phase duringsolidification, there is the potential to more completely homogenize thematerial during heat treatment, which can decrease the likelihood ofstress corrosion cracking (fewer gradients in the homogenized material).If the secondary phase is not continuous, long notches from corrosionare less likely.

In another embodiment of nucleation control, the functionalized surfacemay fully or partially dissolve in the melt and undergo a reaction withmaterials in the melt to form precipitates or inclusions, which may actin the same manner as the nanoparticles in the preceding paragraph. Forexample, titanium particles may be coated on an aluminum alloy particle,which upon melting would dissolve the titanium. However, on cooling thematerial undergoes a peritectic reaction, forming aluminum-titaniumintermetallic (Al₃Ti) inclusions which would serve as nucleation sites.

In another embodiment, the coating may react with impurities to formnucleation sites. An example is a magnesium coating on a titanium alloypowder. Titanium has a very high solubility of oxygen (a commonatmospheric contaminant), which can affect the overall properties. Acoating of magnesium reacts within the melt, binding to dissolved oxygenwhich forms magnesium oxide (MgO) inclusions, promoting nucleation.

Nucleation control may include the use of ceramic particles. In someembodiments, the ceramic particles can be wet by the molten material,while in other embodiments, the ceramic particles cannot be wet by themolten material. The ceramic particles may be miscible or immisciblewith the molten state. The ceramic particles may be incorporated intothe final solid material. In some embodiments, the ceramic particles arerejected from the solid. Exemplary ceramic materials include (but arenot limited to) SiC, HfC, TaC, ZrC, NbC, WC, TiC, TiCo_(0.7)N_(0.3), VC,B₄C, TiB₂, HfB₂, TaB₂, ZrB₂, WB₂, NbB₂, TaN, HfN, BN, ZrN, TiN, NbN, VN,Si₃N₄, Al₂O₃, MgAl₂O₃, HfO₂, ZrO₂, Ta₂O₅, TiO₂, SiO₂, and oxides ofrare-earth elements Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho Er, Tm,Yb, and/or Lu.

Nucleation control may include the use of metallic particles. In someembodiments, the metallic particles can be wet by the molten material.The metallic particles may form an alloy with the molten materialthrough a eutectic reaction or peritectic reaction. The alloy may be anintermetallic compound or a solid solution. In some embodiments, themetallic particles cannot be wet by the molten material and cannot forman alloy with the molten material. Exemplary metallic materials include(but are not limited to) Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Ru, Rh, Pd, La,Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho Er, Tm, Yb, Lu, Ta, W, Re, Os, Ir,Pt, Si, or B.

Nucleation control may include the use of plastic particles. In someembodiments, the plastic particles can be wet by the molten material,while in other embodiments, the plastic particles cannot be wet by themolten material.

Nanoparticles promote surface growth of crystals that have goodepitaxial fit. Nucleation on the surface of a nanoparticle is morelikely when there is good fit between the crystal lattice parameters ofthe nanoparticles and the solidifying material. Nanoparticles may beselected to promote nucleation of a specific phase in the melt.

Generally, nucleation-promoting chemical reactions are dependent on theselected surface functionalization and on the heating (or cooling)parameters.

As nanoparticles or microparticles are organized on a particle surfaceunder conditions for which rapid melting or near melting occurs andrapidly fuses the particles together with very little melt convection,the coating will not have the time or associated energy to diffuse awayfrom its initial position relative to the other powders. This would inturn create a three-dimensional network structure of inclusions. Thus, amethod is provided to control maximum grain size and/or to design apredictable microstructure. The microstructure is dependent on theinitial powder size, shape, and packing configuration/density. Adjustingthe coating and powder parameters allows control of this hierarchicalstructure. In some embodiments, these architectures significantlyimprove material properties by impeding, blocking, or redirectingdislocation motion in specific directions, thereby reducing oreliminating failure mechanisms.

Utilizing the appropriate functionalization, the heat flow duringsolidification may be controlled using heats of fusion or vaporization.In some embodiments, inclusions are pulled into the melt or reactedwithin the melt (as described above). In some embodiments, a coating isrejected to the surface of the melt pool. Utilizing a functionalizationsurface with a high vapor pressure at the desired melting point of thepowder, vaporization would occur, resulting in a cooling effect in themelt which increases the freezing rate. As described above, magnesium ona titanium alloy may accomplish this, in addition to forming oxideinclusions. The effect of this is detectible when comparingnon-functionalized powders to functionalized powders under identicalconditions, as well as comparing the composition of feed material versusthe composition of the final product.

In another embodiment, the opposite effect occurs. Some systems mayrequire slower solidification times than can be reasonably provided in acertain production system. In this instance, a higher-melting-pointmaterial, which may for example be rejected to the surface, freezes.This releases the heat of fusion into the system, slowing the total heatflux out of the melt. Heat may also be held in the melt to slowsolidification by incorporating a secondary material with asignificantly higher heat capacity.

In another embodiment, the heat of formation is used to control heatflow during melt pool formation and/or solidification. For example,nickel microparticles may be decorated with aluminum nanoparticles. Uponsupply of enough activation energy, the exothermic reaction of Ni and Alto NiAl is triggered. In this case, a large heat of formation isreleased (−62 kJ/mol) which may aid in melting the particles fully orpartially. The resulting NiAl intermetallic is absorbed into the meltand stays suspended as a solid (a portion may be dissolved) due to itshigher melting point, thereby acting as a nucleation site as well ashaving a strengthening effect on the alloy later.

Thermodynamic control of solidification may utilizenanoparticles/microparticles or surface coatings which undergo a phasetransformation that is different from phase transformations in the basematerial. The phase transformations may occur at different solidusand/or liquidus temperatures, at similar solidus and/or liquidustemperatures, or at the same solidus and/or liquidus temperatures. Thephase-transformed nanoparticles/microparticles or surface coatings maybe incorporated into the final solid material, or may be rejected fromthe final solid material, or both of these. The phase-transformednanoparticles/microparticles or surface coatings may be miscible orimmiscible with the molten state. The phase-transformednanoparticles/microparticles or surface coatings may be miscible orimmiscible with the solid state.

Thermodynamic control of solidification may utilizenanoparticles/microparticles or surface coatings which vaporize orpartially vaporize. For example, such coatings may comprise organicmaterials (e.g., waxes, carboxylic acids, etc.) or inorganic salts(e.g., MgBr₂, ZnBr₂, etc.)

Thermodynamic control of solidification may utilizenanoparticles/microparticles or surface coatings which release or absorbgas (e.g., oxygen, hydrogen, carbon dioxide, etc.).

Thermodynamic control of solidification may utilizenanoparticles/microparticles or surface coatings with different heatcapacities than the base material.

In addition to controlling the energy within the system, it also ispossible to control the rate at which heat leaves the system bycontrolling thermal conductivity or emissivity (thermal IR radiation).This type of control may be derived from a rejection to the surface orfrom the thermal conductivity of a powder bed during additivemanufacturing, for instance. In one embodiment, the functionalizationmay reject to the surface a low-conductivity material, which may be thefunctionalization material directly or a reaction product thereof, whichinsulates the underlying melt and decreases the freezing rate. In otherembodiments, a layer may have a high/low emissivity which wouldincrease/decrease the radiative heat flow into or out of the system.These embodiments are particularly applicable in electron-beam systemswhich are under vacuum and therefore radiation is a primary heat-flowmechanism.

Additionally, in laser sintering systems, the emissivity of a rejectedlayer may be used to control the amount of energy input to the powderbed for a given wavelength of laser radiation. In another embodiment,the functionalized surface may be fully absorbed in the melt yet theproximity to other non-melted functionalized powders, such as additivemanufacturing in a powder bed, may change the heat conduction out of thesystem. This may manifest itself as a low-thermal-conductivity basepowder with a high-conductivity coating.

Thermal conductivity or emissivity control of solidification may utilizenanoparticles/microparticles or surface coatings which are higher inthermal conductivity compared to the base material. Thenanoparticles/microparticles or surface coatings may be incorporatedinto the melt, or may be rejected, such as to grain boundaries or to thesurface of the melt. The nanoparticles/microparticles or surfacecoatings may be miscible or immiscible with the molten state. Thenanoparticles/microparticles or surface coatings may be miscible orimmiscible with the final solid state.

Thermal conductivity or emissivity control of solidification may utilizenanoparticles/microparticles or surface coatings which are lower inthermal conductivity compared to the base material.

Thermal conductivity or emissivity control of solidification may utilizenanoparticles/microparticles or surface coatings which are higher inemissivity compared to the base material.

Thermal conductivity or emissivity control of solidification may utilizenanoparticles/microparticles or surface coatings which are lower inemissivity compared to the base material.

In some embodiments, the functionalization material may react withcontaminants in the melt (e.g., Mg—Ti—O system). When thefunctionalization material is properly chosen, the reacted material maybe selected such that the formed reaction product has a high surfacetension with the liquid, such that it may be rejected to the surface.The rejected reaction product may take the form of an easily removablescale. Optionally, the rejected layer is not actually removed but ratherincorporated into the final product. The rejected layer may manifestitself as a hard-facing carbide, nitride, or oxide coating, a softanti-galling material, or any other functional surface which may improvethe desired properties of the produced material. In some cases, therejected surface layer may be of a composition and undergo a coolingregime which may result in an amorphous layer on the surface of thesolidified material. These surface-rejected structures may result inimproved properties related to, but not limited to, improved corrosionresistance, stress corrosion crack resistance, crack initiationresistance, overall strength, wear resistance, emissivity, reflectivity,and magnetic susceptibility.

Through contaminant removal or rejection, several scenarios arepossible. Nanoparticles/microparticles or surface coatings that reactwith or bind to undesired contaminants may be incorporated into thesolidification, in the same phase or a separate solid phase. The reactednanoparticles/microparticles or surface coatings may be rejected duringsolidification. When portions or select elements present in thenanoparticles/microparticles or coatings react with or bind tocontaminants, such portions or elements may be incorporated and/orrejected.

In some embodiments, the functionalized surface reacts upon heating toform a lower-melting-point material compared to the base material, suchas through a eutectic reaction. The functionalized surface may be chosenfrom a material which reacts with the underlying powder to initiatemelting at the particle surface, or within a partial volume of theunderlying powder. A heat source, such as a laser or electron beam, maybe chosen such that the energy density is high enough to initiate thesurface reaction and not fully melt the entire functionalized powder.This results in an induced uniform liquid phase sintering at theparticle surface. Upon freezing, the structure possesses acharacteristic microstructure indicating different compositions andgrain nucleation patterns around a central core of stock powder with amicrostructure similar to the stock powder after undergoing a similarheat treatment. This structure may later be normalized or undergopost-processing to increase density or improve the properties.

Another possible reaction is a peritectic reaction in which onecomponent melts and this melted material diffuses into a secondnanoparticle or microparticle, to form an alloyed solid. This newalloyed solid may then act as a phase-nucleation center, or may limitmelting just at the edge of particles.

Incorporating nanoparticles into a molten metal may be challenging whenthe nanoparticles have a thin oxide layer at the surface, since liquidmetals typically do not wet oxides well. This may cause thenanoparticles to get pushed to the surface of the melt. One way toovercome the oxide layer on nanoparticles, and the associatedwettability issues, is to form the nanoparticles in situ during meltpool formation. This may be achieved by starting with nanoparticles ofan element that forms an intermetallic with one component of the basealloy, while avoiding dissolution of the nanoparticles in the melt.Alternatively, binary compound nanoparticles that disassociate atelevated temperatures, such as hydrides or nitrides, may be used sincethe disassociation reaction annihilates any oxide shell on thenanoparticle.

As noted above, the surface functionalization may be designed to bereacted and rejected to the surface of the melt pool. In embodimentsemploying additive manufacturing, layered structures may be designed. Insome embodiments, progressive build layers and hatchings may be heatedsuch that each sequential melt pool is heated long enough to reject thesubsequent rejected layer, thereby producing a build with an externalscale and little to no observable layering within the build of therejected materials. In other embodiments, particularly those whichresult in a functional or desired material rejected to the surface,heating and hatching procedures may be employed to generate a compositestructure with a layered final product. Depending on the buildparameters, these may be randomly oriented or designed, layeredstructures which may be used to produce materials with significantlyimproved properties.

Architected microstructures may be designed in which feature sizes(e.g., distance between nanoparticle nodes) within the three-dimensionalnetwork are selected, along with targeted compositions, for an intendedpurpose. Similarly, layered composite structures may be designed inwhich feature sizes (e.g., layer thicknesses or distance between layers)are selected, along with targeted compositions, for an intended purpose.

Note that rejection to the surface is not necessarily required togenerate layered structures. Functionalized surfaces may be relativelyimmobile from their initial position on the surface of the base powder.During melting, these functionalized surfaces may act as nucleationsites, as previously mentioned; however, instead of absorption into themelt, they may initiate nucleation at the location which was previouslyoccupied by the powder surface and is not molten. The result is afine-grained structure evolving from the surface nucleation source,towards the center. This may result in a designed composite structurewith enhanced properties over the base material. In general, thismechanism allows for the ability to control the location of desiredinclusions through controlled solidification.

In the additive manufacturing of titanium alloys, the problem ofmicrostructural texturing of subsequent layers of molten metals inducesanisotropic microstructures and thus anisotropic structural properties.Dispersing stable ceramic nanoparticles in the solidifying layers mayproduce grain structures with isotropic features which are stable uponrepetitive heating cycles. An example is a stable high-temperatureceramic nanoparticle, such as Al₂O₃ or TiCN attached to the surface of aTi-6Al-4V microparticle powder which is subsequently melted, solidified,and then reheated as the next layer of powder is melted on top. Theceramic nanoparticles can induce nucleation of small grains and preventcoarse grains from forming in the direction of the thermal gradient.

Any solidification control method which derives its primaryfunctionality from the surface functionalization of a powdered materialcan be considered in the scope of this invention. Other methods ofcontrol may include multiple types of control described above. Anexample of a combination of methods includes utilizing rejection to thesurface, internal reaction, along with emissivity control. For instance,a part may be processed using additive manufacturing in which afunctionalization material is selected to be dissolved into the surface,and reacts to form an insoluble material which is rejected to thesurface of the melt pool. This rejected material may then have a lowemissivity, which reflects any additional laser radiation, therebydecreasing the local heating and cooling the material quickly to controlsolidification. The resulting structure is a material with a controlledsolidification structure with a low-emissivity surface coating.

In some embodiments, the solid state is a three-dimensionalmicrostructure containing the nanoparticles and/or microparticles asinclusions distributed throughout the solid state.

In some embodiments, the solid state is a layered microstructurecontaining one or more layers comprising the nanoparticles and/ormicroparticles.

The method may further include creating a structure through one or moretechniques selected from the group consisting of additive manufacturing,injection molding, pressing and sintering, capacitive dischargesintering, and spark plasma sintering. The present invention provides asolid object or article comprising a structure produced using such amethod.

Some variations provide a structure created from the functionalizedpowder via additive manufacturing. The functionalized powder (withnanoparticles/microparticles or surface coating) may be incorporatedinto the final structure. In some embodiments, thenanoparticles/microparticles or surface coating are rejected, creating ascale. The scale may be unbonded to the structure. In some embodiments,the scale bonds to the structure or otherwise cannot be readily removed.This may be advantageous, such as to provide a structuralenhancement—for instance, rejected ceramic particles may add a hardfacing to the final structure. Rejected nanoparticles/microparticles orsurface coating may form a multilayer composite, wherein each layer hasa different composition. In some embodiments, rejectednanoparticles/microparticles or surface coating forms a spatially gradedcomposition within the bulk of the structure. A three-dimensionalarchitecture may also develop in the final microstructure.

Some variations provide a solid object or article comprising at leastone solid phase (i) containing a powdered material as described, or (ii)derived from a liquid form of a powdered material as described. Thesolid phase may form from 0.25 wt % to 100 wt % of the solid object orarticle, such as about 1 wt %, 5 wt %, 10 wt %, 25 wt %, 50 wt %, or 75wt % of the solid object or article, for example.

Other variations provide a solid object or article comprising acontinuous solid phase and a three-dimensional network of nanoparticleand/or microparticle inclusions distributed throughout the continuoussolid phase, wherein the three-dimensional network blocks, impedes, orredirects dislocation motion within the solid object or article.

In some embodiments, the nanoparticle and/or microparticle inclusionsare distributed uniformly throughout the continuous solid phase. Thenanoparticle and/or microparticle inclusions may be present at aconcentration from about 0.1 wt % to about 50 wt % of the solid objector article, such as about 1, 2, 5, 10, 15, 20, 25, 30, 35, 40, or 45 wt%, for example.

In some embodiments, light elements are incorporated into the system.For example, the particle surface (or the surface of nanoparticles ormicroparticles present on the powder particles) may be surface-reactedwith an element selected from the group consisting of hydrogen, oxygen,carbon, nitrogen, boron, sulfur, and combinations thereof. For example,reaction with hydrogen gas may be carried out to form a metal hydride.Optionally, the particle or a particle coating further contains a salt,carbon, an organic additive, an inorganic additive, or a combinationthereof. Certain embodiments utilize relatively inert carbides that areincorporated (such as into steel) with fast melting and solidification.

Methods of producing surface-functionalized powder materials aregenerally not limited and may include immersion deposition, electrolessdeposition, vapor coating, solution/suspension coating of particles withor without organic ligands, utilizing electrostatic forces and/or Vander Waals forces to attach particles through mixing, and so on. U.S.patent application Ser. No. 14/720,757 (filed May 23, 2015), U.S. patentapplication Ser. No. 14/720,756 (filed May 23, 2015), and U.S. patentapplication Ser. No. 14/860,332 (filed Sep. 21, 2015), each commonlyowned with the assignee of this patent application, are herebyincorporated by reference herein. These disclosures relate to methods ofcoating certain materials onto micropowders, in some embodiments.

For example, as described in U.S. patent application Ser. No.14/860,332, coatings may be applied using immersion deposition in anionic liquid, depositing a more-noble metal on a substrate of aless-noble, more-electronegative metal by chemical replacement from asolution of a metallic salt of the coating metal. This method requiresno external electric field or additional reducing agent, as withstandard electroplating or electroless deposition, respectively. Themetals may be selected from the group consisting of aluminum, zirconium,titanium, zinc, nickel, cobalt copper, silver, gold, palladium,platinum, rhodium, titanium, molybdenum, uranium, niobium, tungsten,tin, lead, tantalum, chromium, iron, indium, rhenium, ruthenium, osmium,iridium, and combinations or alloys thereof.

Organic ligands may be reacted onto a metal, in some embodiments.Organic ligands may be selected from the group consisting of aldehydes,alkanes, alkenes, silicones, polyols, poly(acrylic acid),poly(quaternary ammonium salts), poly(alkyl amines), poly(alkylcarboxylic acids) including copolymers of maleic anhydride or itaconicacid, poly(ethylene imine), poly(propylene imine),poly(vinylimidazoline), poly(trialkylvinyl benzyl ammonium salt),poly(carboxymethylcellulose), poly(D- or L-lysine), poly(L-glutamicacid), poly(L-aspartic acid), poly(glutamic acid), heparin, dextransulfate, 1-carrageenan, pentosan polysulfate, mannan sulfate,chondroitin sulfate, and combinations or derivatives thereof.

The reactive metal may be selected from the group consisting of alkalimetals, alkaline earth metals, aluminum, silicon, titanium, zirconium,hafnium, zinc, and combinations or alloys thereof. In some embodiments,the reactive metal is selected from aluminum, magnesium, or an alloycontaining greater than 50 at % of aluminum and/or magnesium.

Some possible powder metallurgy processing techniques that may be usedinclude but are not limited to hot pressing, low-pressure sintering,extrusion, metal injection molding, and additive manufacturing.

The final article may have porosity from 0% to about 75%, such as about5%, 10%, 20%, 30%, 40%, 50%, 60%, or 70%, in various embodiments. Theporosity may derive from space both within particles (e.g., hollowshapes) as well as space outside and between particles. The totalporosity accounts for both sources of porosity.

The final article may be selected from the group consisting of asintered structure, a coating, a weld filler, a billet, an ingot, anet-shape part, a near-net-shape part, and combinations thereof. Thearticle may be produced from the coated reactive metal by a processcomprising one or more techniques selected from the group consisting ofhot pressing, cold pressing, sintering, extrusion, injection molding,additive manufacturing, electron-beam melting, selective lasersintering, pressureless sintering, and combinations thereof.

In some embodiments of the invention, the coated particles are fusedtogether to form a continuous or semi-continuous material. As intendedin this specification, “fused” should be interpreted broadly to mean anymanner in which particles are bonded, joined, coalesced, or otherwisecombined, at least in part, together. Many known techniques may beemployed for fusing together particles.

In various embodiments, fusing is accomplished by sintering, heattreatment, pressure treatment, combined heat/pressure treatment,electrical treatment, electromagnetic treatment, melting/solidifying,contact (cold) welding, solution combustion synthesis, self-propagatinghigh-temperature synthesis, solid state metathesis, or a combinationthereof.

“Sintering” should be broadly construed to mean a method of forming asolid mass of material by heat and/or pressure without melting theentire mass to the point of liquefaction. The atoms in the materialsdiffuse across the boundaries of the particles, fusing the particlestogether and creating one solid piece. The sintering temperature istypically less than the melting point of the material. In someembodiments, liquid-state sintering is used, in which some but not allof the volume is in a liquid state.

When sintering or another heat treatment is utilized, the heat or energymay be provided by electrical current, electromagnetic energy, chemicalreactions (including formation of ionic or covalent bonds),electrochemical reactions, pressure, or combinations thereof. Heat maybe provided for initiating chemical reactions (e.g., to overcomeactivation energy), for enhancing reaction kinetics, for shiftingreaction equilibrium states, or for adjusting reaction networkdistribution states.

Some possible powder metallurgy processing techniques that may be usedinclude, but are not limited to, hot pressing, sintering, high-pressurelow-temperature sintering, extrusion, metal injection molding, andadditive manufacturing.

A sintering technique may be selected from the group consisting ofradiant heating, induction, spark plasma sintering, microwave heating,capacitor discharge sintering, and combinations thereof. Sintering maybe conducted in the presence of a gas, such as air or an inert gas(e.g., Ar, He, or CO₂), or in a reducing atmosphere (e.g., H₂ or CO).

Various sintering temperatures or ranges of temperatures may beemployed. A sintering temperature may be about, or less than about, 100°C., 200° C., 300° C., 400° C., 500° C., 600° C., 700° C., 800° C., 900°C., or 1000° C.

A sintering temperature is preferably less than the reactive-metalmelting temperature. In some embodiments, a sintering temperature may beless than a maximum alloy melting temperature, and further may be lessthan a minimum alloy melting temperature. In certain embodiments, thesintering temperature may be within the range of melting points for aselected alloy. In some embodiments, a sintering temperature may be lessthan a eutectic melting temperature of the particle alloy.

At a peritectic decomposition temperature, rather than melting, a metalalloy decomposes into another solid compound and a liquid. In someembodiments, a sintering temperature may be less than a peritecticdecomposition temperature of the metal alloy. If there are multipleeutectic melting or peritectic decomposition temperatures, a sinteringtemperature may be less than all of these critical temperatures, in someembodiments.

In some embodiments pertaining to aluminum alloys employed in themicroparticles, the sintering temperature is preferably selected to beless than about 450° C., 460° C., 470° C., 480° C., 490° C., or 500° C.The decomposition temperature of eutectic aluminum alloys is typicallyin the range of 400-600° C. (Belov et al., Multicomponent PhaseDiagrams: Applications for Commercial Aluminum Alloys, Elsevier, 2005),which is hereby incorporated by reference herein.

A solid article may be produced by a process selected from the groupconsisting of hot pressing, cold pressing and sintering, extrusion,injection molding, additive manufacturing, electron beam melting,selected laser sintering, pressureless sintering, and combinationsthereof. The solid article may be, for example, a coating, a coatingprecursor, a substrate, a billet, an ingot, a net shape part, a near netshape part, or another object.

EXAMPLES Example 1: Production of AlSi10Mg—WC Functionally Graded MetalMatrix Nanocomposite

In this example, a functionally graded metal matrix nanocomposite isproduced, with AlSi10Mg alloy and tungsten carbide (WC) nanoparticles.The starting AlSi10Mg alloy has an approximate composition of 10 wt %silicon (Si), 0.2-0.45 wt % magnesium (Mg), and the remainder aluminum(Al) except for impurities (e.g., Fe and Mn). The density of tungstencarbide 15.6 g/cm³ and the density of AlSi10Mg is 2.7 g/cm³. Thetungsten carbide nanoparticles have a typical particle size of 15 nm to250 nm.

Tungsten carbide nanoparticles are assembled on an AlSi10Mg alloypowder. This material is consolidated under 300 MPa compaction force andthen melted in an induction heater at 700° C. for one hour. Theresulting material (FIG. 10) exhibits a functional gradient according tothe distribution of WC nanoparticles. FIG. 10 is an SEM image of across-section (side view) of the resulting AlSi10Mg—WC functionallygraded metal matrix nanocomposite.

This is an example of density-driven phase separation of high-densitytungsten carbide nanoparticles segregated to the bottom of a matrix ofaluminum alloy. Melting of the ingot induces spontaneous segregation ofthe WC nanoparticles, of higher density than the AlSi10Mg, to the bottomof the melt; and voids, of lower density than the AlSi10Mg, to the topof the melt. The induction melting of the predistributed ingot preservesthe integrity and dispersion of the WC nanoparticles and mitigatesreaction between the nanoparticles and the AlSi10Mg matrix, preventingsignificant agglomeration of nanoparticles.

Example 2: Production of AlSi10Mg—WC Master Alloy Metal MatrixNanocomposite

In this example, a master alloy metal matrix nanocomposite is produced,with AlSi10Mg alloy and tungsten carbide (WC) nanoparticles.

A functionally graded metal matrix nanocomposite is first producedaccording to Example 1. The material shown in FIG. 10 is the precursorto the master alloy. According to FIG. 10, the tungsten carbidenanoparticles are preferentially located (functionally graded) towardthe bottom of the structure. This is also analogous to the schematic ofFIG. 6. The AlSi10Mg alloy (metal matrix phase) toward the top containslittle or no tungsten carbide nanoparticles. The desired material forthis master alloy is the lower phase, containing a higher volume oftungsten carbide nanoparticles distributed within the AlSi10Mg phase.

The AlSi10Mg alloy (metal matrix phase) labeled “AlSi” is then separatedfrom the lower phase labeled “AlSi+WC”. The resulting material is amaster alloy metal matrix nanocomposite. FIG. 11 is an SEM image of across-section (side view) of the microstructure of the resultingAlSi10Mg—WC master alloy metal matrix nanocomposite. There is awell-distributed network of WC nanoparticles in a high-volume-fractionnanocomposite without significant nanoparticle accumulation.

This master alloy metal matrix nanocomposite example of AlSiMg alloywith a hard reinforcement phase of tungsten carbide nanoparticlesdemonstrates the use of a pre-dispersed ingot in the process ofdensity-driven phase separation. The total volume fraction of WC tometal matrix is increased from the pre-dispersed ingot by phasesegregation.

Limitations in cost, availability, and performance impede progress ofmetal matrix composites across several industries. Variations of thisinvention provide an efficient, low-cost route to manufacturing metalmatrix nanocomposites. The versatility of this method enables systems ofreinforcement and metal matrix composite components to be manufacturedwith a high performance potential in many different applications.

Commercial applications include high-wear-resistant alloy systems,creep-resistant alloys, high-temperature alloys with improved mechanicalproperties, high thermal-gradient applications, radiation-tolerantalloys, high-conductivity and high-wear-resistant injection moldingdies, turbine disks, automotive and aviation exhaust system components,and nuclear shielding, for example. This invention providesnear-net-shape casting of objects with complex surfaces, maintainingfunctionally graded reinforcement across the designed surfaces.Density-driven phase separation in casting can result in thickfunctionally graded products.

Other specific applications may include gearing applications where thefunctional gradient acts as a case hardening; pistons with hard facingfor improved wear and thermal behavior; high-conductivity,wear-resistant tooling; rotating fixtures such as shafts and couplers;engine valves; cast structures of lightweight metals; high-conductivitystructural materials; wear-resistant materials; impact surfaces;creep-resistant materials; corrosion-resistant materials; and highelectrical-conductivity metals.

In this detailed description, reference has been made to multipleembodiments and to the accompanying drawings in which are shown by wayof illustration specific exemplary embodiments of the invention. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatmodifications to the various disclosed embodiments may be made by askilled artisan.

Where methods and steps described above indicate certain eventsoccurring in certain order, those of ordinary skill in the art willrecognize that the ordering of certain steps may be modified and thatsuch modifications are in accordance with the variations of theinvention. Additionally, certain steps may be performed concurrently ina parallel process when possible, as well as performed sequentially.

All publications, patents, and patent applications cited in thisspecification are herein incorporated by reference in their entirety asif each publication, patent, or patent application were specifically andindividually put forth herein.

The embodiments, variations, and figures described above should providean indication of the utility and versatility of the present invention.Other embodiments that do not provide all of the features and advantagesset forth herein may also be utilized, without departing from the spiritand scope of the present invention. Such modifications and variationsare considered to be within the scope of the invention defined by theclaims.

What is claimed is:
 1. A method of making a metal nanocomposite, said method comprising: (a) providing a precursor composition comprising metal-containing microparticles and nanoparticles, wherein said nanoparticles are chemically and/or physically disposed on surfaces of said microparticles; (b) consolidating said precursor composition into an intermediate composition comprising said metal-containing microparticles and said nanoparticles, wherein said nanoparticles are consolidated in a three-dimensional architecture throughout said intermediate composition; and (c) processing said intermediate composition to convert said intermediate composition into a metal nanocomposite.
 2. The method of claim 1, wherein said precursor composition is in powder form.
 3. The method of claim 1, wherein said intermediate composition is in ingot form.
 4. The method of claim 1, wherein said microparticles contain an element selected from the group consisting of Al, Mg, Ni, Fe, Cu, Ti, V, Si, and combinations thereof.
 5. The method of claim 1, wherein said nanoparticles contain a compound selected from the group consisting of metals, ceramics, cermets, intermetallic alloys, oxides, carbides, nitrides, borides, polymers, carbon, and combinations thereof.
 6. The method of claim 1, wherein step (b) includes pressing, binding, sintering, or a combination thereof.
 7. The method of claim 1, wherein step (c) includes pressing, sintering, mixing, dispersing, friction stir welding, extrusion, binding, melting, semi-solid melting, capacitive discharge sintering, casting, or a combination thereof.
 8. The method of claim 1, wherein said metal nanocomposite has a cast microstructure.
 9. The method of claim 1, wherein said metal-containing microparticles and said nanoparticles are each dispersed throughout said metal nanocomposite.
 10. The method of claim 1, wherein said metal nanocomposite has a layered configuration including at least a first layer comprising said nanoparticles and at least a second layer comprising said metal-matrix phase. 